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Effects of surface properties of (010), (001) and (100) of MnWO4 and FeWO4 on absorption of collector
Applied Surface Science 367 (2016) 354–361
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Effects of surface properties of (010), (001) and (100) of MnWO4 and FeWO4 on absorption of collector X.Y. Qiu a,b , H.W. Huang a,b,∗ , Y.D. Gao b a b
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, Hunan, China Guangzhou Research Institute of Non-Ferrous Metals, Guangzhou 510651, Guangdong, China
a r t i c l e
i n f o
Article history: Received 13 June 2015 Received in revised form 19 January 2016 Accepted 21 January 2016 Available online 23 January 2016 Keywords: DFT Surface stability Huebnerite Ferberite Wolframite Cleavage plane
a b s t r a c t The atom distribution and electronic properties of (010), (001) and (100) planes of MnWO4 and FeWO4 were studied based on a DFT calculation. The surface stabilities of the three planes were compared according to their surface energies. The most stable one is (010) plane, followed by (001) and (100). (010) and (001) are the main planes for absorption of anion collector ions, which is supported by their bonding relationship and charge density distribution of surface atoms and finally proved by the results of flotation test and stereomicroscope analysis. In addition, the tungsten atoms can be viewed as the absorption site for collectors in (001) plane but not in (010) plane, which can explain the phenomenon in flotation test that the recovery of wolframite can hardly be further boosted even with a high dosage of BHA. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tungsten is an important sort of strategic resource and it usually occurs as the wolframite in the nature. At present, gravity and magnetic separation are still the main methods for recovering wolframite [1], but they have deficiency in recovering fine and low-grade ores. Since a series of efficient flotation reagents were developed, flotation is convenient in fine and low-grade fraction separation. Chelating reagent and fatty acid are widely used in flotation of oxide ores [mineral processing] and they are commonly used as collectors of wolframite [2]. The mechanisms of absorption of these collectors on wolframite surface have been discussed to some extent. Wang et al. [3] studied the flotation behavior of wolframite with different Mn/Fe ratio with 1-Nitroso-2-naphthol and octanohydroxamic acid. They considered the two kinds of chelating collectors had a more intensive interaction with Mn than Fe on wolframite surface. And this opinion was further specified by Hu et al. [2]. The hydrophobic agglomeration and spherical agglomeration of wolframite fines were also involved [4,5]. In addition, a study [6] reported the absorption of sodium oleate (NbOl) and benzohy-
∗ Corresponding author at: School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, Hunan, China. E-mail address: [email protected] (H.W. Huang). http://dx.doi.org/10.1016/j.apsusc.2016.01.190 0169-4332/© 2016 Elsevier B.V. All rights reserved.
droxamic acid (BHA) on wolframite, and indicated that high iron wolframite showed higher flotation with BHA as a collector while lower flotation with NaOl as a collector, but the high manganese wolframite displayed an opposite behavior. In terms of previous researches, it can be concluded that the iron and manganese atoms in the surface of wolframite should be the absorption sites when anionic collectors are used. However, they have not referred to the precondition for absorption of collectors, namely, whether the wolframite has a proper surface crystal structure for absorption of collectors is not involved. As we know, besides of iron, manganese and oxygen, tungsten is also a component of wolframite. Whether tungsten in the surface can be absorbed by collector ions should also be taken into account. For this reason, the research of crystal surface properties of wolframite is indispensable for explaining the phenomenon of collector absorption. (010), (100) and (001) are the most common cleavage planes which can be seen from wolframite crystals. According to the mineralogical characters of wolframite, these planes are easy to be formed when cracked by force. Therefore, their surface properties should be related to the absorption of collector ions, and then, to the flotation behavior of wolframite. Actually, wolframite is an isomorphous series with doped iron or manganese atoms between huebnerite (MnWO4 ) and ferbnerite (FeWO4 ). The Mn/Fe ratio of wolframite is variable and there is little evidence of the presence of pure MnWO4 or FeWO4 in the nature.
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Table 1 Experimental and calculating lattice parameters of MnWO4 and FeWO4 . Lattice parameters
Experimental values MnWO4
˚ Lengths (A)
a b c
Angles (◦ )
ˇ
4.8277 (8) [12] 5.761 (1) [12] 4.997 (8) [12] 91.14 [12]
FeWO4 4.73 [12] 5.703 [12] 4.952 [12] 90.00 [12]
That is to say, it is difficult to obtain general rules of micro surface features of wolframite from natural minerals by traditional measuring and testing techniques. The simulation techniques of quantum chemistry based on the density function theory (DFT), however, are gradually developed and utilized for the research of crystal microstructure, which allow the atomic and electronic properties of crystal surface to be understood. And these techniques can also be applied in mineral processing to study, for example, the floatability of minerals [7], band structure and density of states (DOS) [8], absorption of micro molecules on surface [9], highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of bonding atoms [8] and so on. In this work, the atom and electron properties and the stability of (010), (001) and (100) surfaces of MnWO4 and FeWO4 have been analyzed with a DFT calculation in the purpose of studying the features of crystal surface structure and their influence on the absorption of anionic collectors. The screening analysis and laboratory flotation test have been carried out and their productions have been also studied for the relationship between cleavage characteristics and flotation behavior by stereomicroscope. 2. Results and discussion 2.1. Computation details 2.1.1. Lattice and slab geometric optimizations The total energy plane-wave pseudopotential based on DFT was used for performing all calculations with the CASTEP
Geometry optimized values
Error (%)
MnWO4
FeWO4
MnWO4
FeWO4
4.855 5.846 5.043
4.71 5.731 4.972
+0.58 +1.47 +0.92
−0.42 +0.49 +0.40
+0.08
+0.58
91.215
90.52
computer code. The BFGS algorithm was recommended to reach the minimum total energy of the system for its stability and efficiency [10,11] with the functional of GGA-PBE (the generalized gradient approximation of Perdew–Burkee–Ernzerhof). The corresponding bulk lattice parameters of first-principles calculation in bulk geometric optimization as well as the experimental values were displayed in Table 1. The spin polarization was taken into account. The cutoff energy for plane-wave expansion was set to 700 eV and Brillouin-zone integration was performed with the 4 × 4 × 4 and 4 × 4 × 1 k-grid using the Monkhorst–Pack method for bulk optimization and surface structure optimization respectively, which could guarantee acceptable calculation errors. When the total energy changes were converged to less than 1 × 10−5 eV/atom, ˚ the stress less than 0.05 GPa, and the the forceless than 0.03 eV/A, ˚ the relaxation of all atoms was displacement less than 0.001 A, finished. The crystals of MnWO4 and FeWO4 are both monoclinic and belong to the space group P2/C. They have similar lattice parameters according to Table 1. The cleavage planes perpendicular to the three coordinate axes x, y and z are the cleavage planes (100), (010) and (001). When cleaving surfaces along a certain orientation we find the atom distribution of each surface is not unique (see in Fig. 1). What is more, the two newly generated surfaces may be symmetrical or non-symmetrical, which will impact the calculations of surface energies. For calculating the surface energies, the symmetrical surfaces are calculated using one slab while the nonsymmetrical surfaces need two. Each slab contains twelve layers of
Fig. 1. Different slab models of three cleavage planes. The red, blue and purple balls respectively denote oxygen tungsten and manganese (iron) atoms. Slabs of (010)-A, (010)-C and (001)-A have symmetrical surfaces while the others are non-symmetrical. (For interpretation of the references to color in this legend, the reader is referred to the web version of the article.)
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Table 2 Surface energies of different cleavage planes. Plane type
2.1.2. Surface energy calculations For symmetrical slabs, such as (010)-A, the surface energy after relaxation will be simply calculated by Eq. (1) [10,14,15],
Surface energy (J/m) MnWO4
FeWO4
(010)-A (010)-B1 (010)-B2 (010)-C
1.3537
2.5143
5.5908
6.8473
0.6926
0.9186
(001)-A (001)-B1 (001)-B2
1.6228
2.8964
4.4784
6.1233
4.0291
4.9334
4.5262
5.3920
(100)-A1 (100)-A2 (100)-B1 (100)-B2
MnWO4 (average)
FeWO4 (average)
2.5457
3.4267
3.0506
4.5099
4.2777
5.1627
Esur =
Erel-slab − nEbulk 2S
(1)
where Erel-slab is the total energy of relaxed slab, Ebulk is the bulk energy, n means the number of bulks contained in a slab, S denotes the surface area and the number 2 means the two surfaces of a slab.
Fig. 2. Bonding relationship among O, W and Mn(Fe) atoms in three slabs before relaxation. The purple, blue and red balls respectively denote the Mn(Fe), W and O atoms. (For interpretation of the references to color in this legend, the reader is referred to the web version of the article.)
atoms. And the four top layers and four bottom layers are relaxed in the vacuum to form a surface state while the four middle layers are constrained to represent the bulk state and this method was particularly discussed by Mastrikov et al. [13].
Fig. 3. Total charge density distribution (A) and charge density difference distribution (B) of MnWO4 and FeWO4 bulk. The blue areas in B denote the loss of charges while the red areas mean the gain of charges. (For interpretation of the references to color in this legend, the reader is referred to the web version of the article.)
Table 3 Bond lengths and Mulliken populations of bulk and relaxed slabs of MnWO4 and FeWO4 . The bond lengths and populations of (001)-A given in the round bracket indicate the two bonds of the same type have different values, where in (010)-A and (010)-C they have the same values. Bond type
Bulk
(010)-A
˚ Bond length (A)
Mn-O-I Mn-O-II Mn-O-III W-O-I W-O-II W-O-III Fe-O-I Fe-O-II Fe-O-III W-O-I W-O-II W-O-III
In experiment
In models
2.1 [19] 2.16 [19] 2.27 [19] 1.79 [19] 1.92 [19] 2.13 [19] 1.86 [20] 2.16 [20] 2.23 [20] 1.7 [20] 1.97 [20] 2.28 [20]
2.132 2.153 2.304 1.812 1.926 2.155 2.085 2.1 2.101 1.822 1.93 2.131
(010)-C
(001)-A
Population
˚ Bond length (A)
Population
˚ Bond length (A)
Population
˚ Bond length (A)
Population
0.26 0.14 0.13 0.74 0.48 0.3 0.27 0.16 0.19 0.7 0.51 0.28
2.379 2.064 2.444 1.821 1.784 – 2.499 1.944 2.152 1.8405 1.764 –
0.14 0.22 0.09 0.63 0.71 – 0.1 0.27 0.16 0.56 0.73 –
2.043 2.109 – 1.799 1.928 2.191 1.947 2.033 – 1.81 1.9346 2.1996
0.24 0.22 – 0.71 0.44 0.27 0.26 0.26 – 0.7 0.41 0.27
2.171 (2.131) 2.124 2.751 (2.07) 1.78 (1.821) 1.839 2.552 (1.812) 2.078 (2.16) 1.998 2.137 (2.018) 1.78 (1.917) 1.833 2.294 (1.817)
0.25 (0.28) 0.19 0.02 (0.24) 0.71 (0.66) 0.65 0.02 (0.65) 0.27 (0.26) 0.21 0.21 (0.22) 0.71 (0.52) 0.65 0.12 (0.67)
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Fig. 4. Distortions and stretches of bond angles and bond lengths of relaxed slabs. The capital letter A, B and C denote (010)-A, (010)-C and (001)-A of MnWO4 while lowercase letter a, b and c represent (010)-A, (010)-C and (001)-A of FeWO4 , respectively.
For non-symmetrical slabs, such as (010)-B1 and (010)-B2, we calculate the surface energies of both slabs with Eq. (1) respectively, and then average them. In terms of the calculation method above, we obtain the surface energies of different slab models and list them in Table 2. According to Table 2, we can draw the following conclusion. Firstly, all the energies of FeWO4 surfaces are slightly higher than the counterparts of MnWO4 . It means that the FeWO4 surfaces have higher activity than MnWO4 surfaces. Secondly, among three types of crystal surfaces, the (010) has the lowest average surface energy, followed by the (001) and (100). Moreover, the surface energies of (010)-C and (010)-A and (001)-A are prominently lower than others’. It implies that these three kinds of surfaces probably appear more frequently in crystals. Finally, considering the surface atom distribution with surface energy, we find that in the (010) and (100) surfaces the energy of surfaces with manganese (or iron) atoms exposed to the vacuum is lightly smaller than one with tungsten atoms exposed, which means the former is more stable than the latter. In the light of Bond’s breakage theory [16], when cracking, the less the surface energy of new formed surface is, the more stable the surface is. Moreover, as the cracked crystal particle size decreases, the specific surface area grows so quickly that the surface energy becomes the crucial influence on the breakage of fine particles. Thus we can speculate that if the surface energies of some crystal cleavages are prominently lower than others, they will be easily generated. We understand that the surface energies of (010) and (001) planes of wolframite are remarkably lower than one of (100)
plane according to the calculated results above, and each two of the three planes are approximately orthogonal in space. Therefore, the cracked particles will be gradually shaped to be platy to generate as many as possible (010) and (001) planes with the decrease of particle size. This speculation will be further confirmed in Section 2.2. In addition, the surface energies of (010)-A, (010)-C and (001)-A are extremely lower than others. It will be discussed in detail. 2.1.3. Bonding and charge density of surface atoms The bonding and charge density of surface atoms of (010)-A, (010)-C and (001)-A of MnWO4 and FeWO4 are analyzed for understanding their influence on absorption. The bonding relationships of slab surface atoms are mapped in Fig. 2. It can be clearly seen that there are only three sorts of bonding ways among W, O, Mn and Fe atoms, and they are W-O, Mn-O and Fe-O. Each unit area of slab surface has equal numbers of breaking bonds, including four breaking bonds of W-O-III in slab (010)-A, four Mn(Fe)-O-III in (010)-C, two W-O-II and two Mn(Fe)-O-II in (001)-A. A slab surface reconstruction has taken place after relaxation, which leads to new bonding relationship. This can be appropriately depicted by bond lengths and Mulliken populations. In terms of these parameters, the bonding ways among W, O, Mn and Fe can be sorted in six types and they are listed in Table 3. W-O has shorter bond lengths but higher population, compared with Mn(Fe)-O, which reflects that the bond strength of W-O is greatly stronger than that of Mn(Fe)-O. Besides, Fe-O is a little more intensive than Mn-O in bond strength. This trend is in accordance with the elemental
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Fig. 5. Charge density difference distribution of three kinds of slabs after relaxation. A, B and C denote (010)-A, (010)-C and (001)-A of MnWO4 while a, b and c represent (010)-A, (010)-C and (001)-A of FeWO4 , respectively.
electronegativity difference where the W has the highest Pauling electronegativity (2.36), followed by Fe (1.83) and Mn (1.55) [17]. The charge density distribution is quite intuitive to reflect the bonding relationship. In Fig. 3, the total charge density distribution and charge density difference distribution of MnWO4 and FeWO4 bulk are respectively drawn to show the interaction between O and other atoms. From Fig. 3A we can realize that W and O are bonding with a high degree of orbital overlap while the degree of orbital overlap between Mn(Fe) and O is quite low. In addition, the Fig. 3 B shows that an obviously intensive gain and loss of charges gather around the area of electronic orbital overlap between W and O, but that cannot be seen in Mn-O or Fe-O. Considering it with the population and bond length together, we reach the conclusion that the bonding nature between W and O is close to covalent bond while Mn-O and Fe-O are tend to be electrovalent bonds. It can also be seen from Fig. 2 and Table 3 that the W and O atoms exposed to vacuum of (010)-A is unsaturated because of the breaking bond W-O-III. And the bonds between unsaturated O and adjacent W, Mn or Fe differ from the counterparts of bulk. For Mn-O-I or Fe-O-I, the bond lengths increase while the populations decrease, but for W-O-II, the situation is just opposite. It demonstrates that the breaking of W-O-III bond, on one hand, enhances the bond W-O-II, but weakens the bond Mn(Fe)-O-I on the other hand. It contributes to the bond nature among W, O, and Mn(Fe) atoms. Since the bond Mn(Fe)-O-I is electrovalent, the existence of unsaturated O atoms will cause the electrons between O and Mn(Fe) more close to O, that, on the contrary, is just more
beneficial to the formation of covalent bond between O and W. However, in slab (010)-C the breaking bonds are Mn(Fe)-O-III. Though it has a reinforced compensation to Mn(Fe)-O-I and Mn(Fe)O-II, it has little effect on bonding between W and O as the bonding interaction from W-O is extremely stronger than one from Mn(Fe)O. For further study of the bonding relationship of surface atoms of (010)-A, (010)-C and (001)-A, we marked out the bond length and angle changes in relaxed slabs and mapped the charge density difference distribution of relaxed slabs (see Figs. 4 and 5). In (010)-A and (010)-C, all the unsaturated Mn, Fe and W atoms have orbitals to accept electrons from anionic collector, but the spatial configuration and bonding effects of W with adjacent O determines W is hard to be an absorption site. Because of the strong bonding interaction between W and O, the surface W even can combine with adjacent four O atoms to form a stable tetrahedral geometry. Due to the distortion of bond angles and shrink of bond length, the O atoms are spatially higher than and block the W atoms. From Fig. 5B and b we can see the part of dxy of W toward to the vacuum is blocked by the orbitals of O atoms which causes W atoms not to bond with collector ions. On the other side, the Mn and Fe atoms are not blocked by O atoms (see Fig. 5A and a), so their unoccupied orbital dz2 can easily accept electrons to form a bond. However, the situation is a little different in (001)-A as it has two types of breaking bonds W-O and Mn(Fe)-O at the same time and the unoccupied orbital of unsaturated W or Mn(Fe) atoms are not blocked by O atoms (see Fig. 5C and c). That means all of them can be the absorption sites.
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Fig. 6. Photos of screening and flotation products under stereo microscope. A, B and C respectively represent three fractions of −74 + 50 m, −50 + 40 m and −40 m of wolframite raw ores. D and E respectively denote the flotation concentrates and tailings from the size fraction of −50 + 40 m feedings.
2.2. Results of flotation test and stereo microscope analysis of flotation production
directly defined as a ratio of dry weights of the concentrate to the feeding,
To verify our speculation of the relationship between particle size and cleavage plane features of wolframite, we classified a batch of pure minerals of wolframite into three size fractions of −74 + 50 m, −50 + 40 m and −40 m by screening, and observed their cleavage plane characteristics by stereomicroscope. The photos of products from three fractions are given in Fig. 6. It can be clearly seen that most of particles from the fraction of −74 + 50 m are granular that they have irregular cleavages. Then the platy fraction of products rises rapidly while the granular reduce with the decline of size. It is worth noting that the majority of platy particles in grades of −50 + 40 m and −40 m have perfect cleavage planes. This suggests that the occurrence frequency of platy particles as well as the perfect cleavage planes gradually increases with the decline of particle size, and that is in line with the speculation of surface energy calculation results. The fraction of −50 + 40 m was chose for flotation test to study the relationship between flotation behavior and cleavages of wolframite with BHA as collector. The effect of dosage of BHA was also involved. The concentrates and tailings from a flotation test with a dosage of 65 mg/L of BHA were both photographed (see in Fig. 6). From Fig. 6 we can see an overwhelming proportion of platy particles with clearly perfect cleavages in concentrates, which is far more than one in tailings and raw ores. It indicates these particles are easier to be gathered and floated into concentrates by collector. The pH of pulp was 7. The terpenic oil was used as frothing agent (16 mg/L) and BHA was used as single collecting agent. The result from Fig. 7 shows that the floatability of wolframite is quite low without BHA and it gradually grows with the dosage growth of BHA. Since the wolframite was pure, the recovery (ε) is
ε=
˛ × 100%, ˇ
where ε, ˛ and ˇ respectively denote the wolframite recovery, dry weight of concentrate and dry weight of feeding. It is worth mentioning that when the recovery of wolframite reaches to about 50%, it can hardly be boosted even using a high dosage of BHA. And this phenomenon can also be seen in Deng. L’s research [18]. What is more, there are still a small number of platy particles with perfect cleavages left in tailings (see in Fig. 6E). According to the calculation results and the conclusion we have, we can easily come to the explanation of these questions above. The (010) cleavage plane is the most common occurrence of wolframite crystals and it also has 100
Wolframite
Recovery (%)
80
60
40
20
0
0
20
40
60
80
100
Dosage of BHA (mg/L) Fig. 7. Relationship between dosage of BHA and recovery of wolframite.
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Fig. 8. Atomic bonding relationships and charge density distribution of BHA absorption models on (010)-A, (010)-C and (001)-A planes before and after geometry optimization.
two types, the (010)-A and (010)-C. If the wolframite particles with cleavages mainly dominated by (010)-A which exposing W atoms without Mn or Fe atom, they can hardly be absorbed by BHA. As it is almost impossible to forbid the random and accessible occurrence of (010)-A in wolframite particles, a part of particles without absorption of BHA are left in tailings, which leads to a low recovery of wolframite. 2.3. Absorption of BHA on different surfaces To further understanding the absorption of BHA on different mineral surfaces, a series of simulations based on the models above were performed. Considering the similarities between MnWO4 and FeWO4 surfaces, only the absorption on (010)-A, (010)-C and (001)A planes of MnWO4 was simulated. The red, dark blue, gray, purple and light blue balls respectively denote O, N, C, Mn and W atoms. A, B and C respectively depict absorption of BHA on (010)-A, (010)-C and (001)-A planes of MnWO4 , while a, b, and c respectively represent their corresponding charge density distribution. Fig. 8 has recorded the main atomic changes of absorption of BHA on different planes before and after geometry optimization and mapped their charge density distribution. We can clearly see obvious absorption occurring on different planes except of (010)A. Fig. 8-A shows that there is no bonding between exposed W atoms of (010)-A and O atoms of BHA as they have never reached to
the effective bonding range (according to Table 3 the bond lengths ˚ and showing no electron density would never be longer than 3 A), enhancement around W and O atoms, Fig. 8-a goes a step further to demonstrate it. On (010)-C plane, the distances between Mn and two O atoms are reduced respectively from 3.128 A˚ and 3.168 A˚ ˚ while on (001)-A plane, from 3.697 A˚ and to 1.970 A˚ and 2.041 A, ˚ And the electron density distribu3.728 A˚ to 2.109 A˚ and 2.298 A. tion of bonding atoms has overlapped. It is worth to note that the absorption on (001)-A plane is based on the bonding among one Mn atom, one W atom and two O atoms, accompanied with a breakage of one inner Mn-O bond (the Mn-O bond has been stretched from 2.134 A˚ to 3.143 A˚ which is longer than effective bonding distance of Mn-O). All these calculating results demonstrate that BHA can absorb on (010)-C and (001)-A planes but not on (010)-A planes. It agrees well with our explanation for the results of pure mineral flotation test in Section 2.2 and strongly support our viewpoints. 3. Conclusion In this study a DFT calculation was carried out to obtain the surface energies, bonding nature, Mulliken population and charge density distribution of the (010), (001) and (100) planes of MnWO4 and FeWO4 , and the cleavage plane characteristics of screening and flotation production were analyzed by stereo microscope. Based on the results of this study, we conclude the following points:
X.Y. Qiu et al. / Applied Surface Science 367 (2016) 354–361
(1) According to the calculation result of surface energy, the (010) plane is the easiest one to be seen in wolframite, followed by (001) plane and (100) plane. (2) The bonding nature and spatial distribution of surface atoms demonstrate that (010) and (001) are the primary planes for the absorption of anion collector ions, in (010) plane of which only Mn and Fe can be the absorption sites while in (001) plane W can also be the absorption site. (3) The stereo microscope observations of screening and flotation productions show that the cleavage plane features of wolframite particles are in accordance with the prediction based on calculation results, namely, the proportion of platy particles along with their perfect cleavage planes increase with the decrease of particle size. (4) When using BHA alone as the collector of wolframite, the recovery of wolframite around 50% can hardly be further boosted even using a high dosage of BHA, that is mainly because a part of cleavage planes of wolframite is occupied by W atoms and hard to be absorbed by collector ions. Acknowledgements This research is supported by High Performance Computing Center of Central South University of China. The authors wish to thank them very much. References [1] Pradip, Recent advances in the recovery of tungsten values in the fine and ultrafine size range, Bull. Mater. Sci. 19 (2) (1996) 267–293. [2] Y. Hu, D. Wang, Z. Xu, A study of interactions and flotation of wolframite with octyl hydroxamate, Miner. Eng. 10 (6) (1997) 623–633. [3] D. Wang, Y. Hu, W. Hu, Flotation behavior of wolframite with different components, J. Central South Inst. Min. Metall. (China) (4) (1986) 40–45. [4] W. Dawei, W. Kewu, Q. Jicun, Hydrophobic agglomeration and spherical agglomeration of wolframite fines, Int. J. Miner. Process. 17 (3) (1986) 261–271.
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[5] G. Kelsall, J. Pitt, Spherical agglomeration of fine wolframite ((Fe,Mn)WO4 ) mineral particles, Chem. Eng. Sci. 42 (4) (1987) 679–688. [6] S. Yang, Q. Feng, X. Qiu, et al., Relationship between flotation and Fe/Mn ratio of wolframite with benzohydroxamic acid and sodium oleate as collectors, Physicochem. Probl. Min. Process. 50 (2) (2014) 747–758. [7] Y. Chen, J. Chen, J. Guo, A DFT study on the effect of lattice impurities on the electronic structures and floatability of sphalerite, Miner. Eng. 23 (14) (2010) 1120–1130. [8] Y. Li, J. Chen, C. Ye, et al., Density functional theory study of influence of impurity on electronic properties and reactivity of pyrite, Trans. Nonferrous Metals Soc. China 21 (8) (2011) 1887–1895. [9] J. Chen, Y. Chen, A first-principle study of the effect of vacancy defects and impurities on the adsorption of O2 on sphalerite surfaces, Colloids Surf. A: Physicochem. Eng. Aspects 363 (1) (2010) 56–63. [10] F. Wang, K. Li, N. Zhou, Structural, electronic properties and stability of AlCMn3 (111) surfaces by first-principles calculations, Appl. Surf. Sci. 289 (36) (2014) 351–357. [11] B.G. Pfrommer, M. Côté, S.G. Louie, et al., Relaxation of crystals with the quasi-Newton method, J. Comput. Phys. 131 (1) (1997) 233–240. [12] J. Macavei, H. Schulz, The crystal structure of wolframite type tungstates at high pressure, Z. Für Krist. Cryst. Mater. 207 (1) (1993) 193–208. [13] Y.A. Mastrikov, E. Heifets, E. Kotomin, et al., Atomic, electronic and thermodynamic properties of cubic and orthorhombic LaMnO3 surfaces, Surf. Sci. 603 (2) (2009) 326–335. [14] Y. Han, Y. Dai, D. Shu, et al., First-principles study of TiB2 (0001) surfaces, J. Phys. Condens. Matter 18 (17) (2006) 4197. [15] W. Liu, C. Wang, J. Cui, et al., Ab initio calculations of the CaTiO3 (111) polar surfaces, Solid State Commun. 149 (43) (2009) 1871–1876. [16] B.A. Wills, Wills’ Mineral Processing Technology: An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery, Butterworth-Heinemann, 2011, pp. 270–276. [17] J.A. Dean, Lange’s Handbook of Chemistry, 15th ed., McGraw-Hill, New York, 2006, pp. 143–147. [18] L. Deng, H. Zhong, S. Wang, et al., A novel surfactant N-(6-(hydroxyamino)-6-oxohexyl) octanamide: synthesis and flotation mechanisms to wolframite, Sep. Purif. Technol. 145 (4) (2015) 8–16. [19] U. Gattermann, S.-H. Park, M. Kaliwoda, Synthesis and characterisation of In-doped MnWO4 -type solid-solutions: Mn1–3x In2x䊐x WO4 (x = 0–0.11), J. Solid State Chem. 219 (2014) 191–200. [20] M.A.P. Almeida, L.S. Cavalcante, C. Morilla-Santos, et al., Electronic structure and magnetic properties of FeWO4 nanocrystals synthesized by the microwave-hydrothermal method, Mater. Charact. 73 (2012) 124–129.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of surface properties of (010), (001) and (100) of MnWO4 and FeWO4 on absorption of collector X.Y. Qiu a,b , H.W. Huang a,b,∗ , Y.D. Gao b a b
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, Hunan, China Guangzhou Research Institute of Non-Ferrous Metals, Guangzhou 510651, Guangdong, China
a r t i c l e
i n f o
Article history: Received 13 June 2015 Received in revised form 19 January 2016 Accepted 21 January 2016 Available online 23 January 2016 Keywords: DFT Surface stability Huebnerite Ferberite Wolframite Cleavage plane
a b s t r a c t The atom distribution and electronic properties of (010), (001) and (100) planes of MnWO4 and FeWO4 were studied based on a DFT calculation. The surface stabilities of the three planes were compared according to their surface energies. The most stable one is (010) plane, followed by (001) and (100). (010) and (001) are the main planes for absorption of anion collector ions, which is supported by their bonding relationship and charge density distribution of surface atoms and finally proved by the results of flotation test and stereomicroscope analysis. In addition, the tungsten atoms can be viewed as the absorption site for collectors in (001) plane but not in (010) plane, which can explain the phenomenon in flotation test that the recovery of wolframite can hardly be further boosted even with a high dosage of BHA. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tungsten is an important sort of strategic resource and it usually occurs as the wolframite in the nature. At present, gravity and magnetic separation are still the main methods for recovering wolframite [1], but they have deficiency in recovering fine and low-grade ores. Since a series of efficient flotation reagents were developed, flotation is convenient in fine and low-grade fraction separation. Chelating reagent and fatty acid are widely used in flotation of oxide ores [mineral processing] and they are commonly used as collectors of wolframite [2]. The mechanisms of absorption of these collectors on wolframite surface have been discussed to some extent. Wang et al. [3] studied the flotation behavior of wolframite with different Mn/Fe ratio with 1-Nitroso-2-naphthol and octanohydroxamic acid. They considered the two kinds of chelating collectors had a more intensive interaction with Mn than Fe on wolframite surface. And this opinion was further specified by Hu et al. [2]. The hydrophobic agglomeration and spherical agglomeration of wolframite fines were also involved [4,5]. In addition, a study [6] reported the absorption of sodium oleate (NbOl) and benzohy-
∗ Corresponding author at: School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, Hunan, China. E-mail address: [email protected] (H.W. Huang). http://dx.doi.org/10.1016/j.apsusc.2016.01.190 0169-4332/© 2016 Elsevier B.V. All rights reserved.
droxamic acid (BHA) on wolframite, and indicated that high iron wolframite showed higher flotation with BHA as a collector while lower flotation with NaOl as a collector, but the high manganese wolframite displayed an opposite behavior. In terms of previous researches, it can be concluded that the iron and manganese atoms in the surface of wolframite should be the absorption sites when anionic collectors are used. However, they have not referred to the precondition for absorption of collectors, namely, whether the wolframite has a proper surface crystal structure for absorption of collectors is not involved. As we know, besides of iron, manganese and oxygen, tungsten is also a component of wolframite. Whether tungsten in the surface can be absorbed by collector ions should also be taken into account. For this reason, the research of crystal surface properties of wolframite is indispensable for explaining the phenomenon of collector absorption. (010), (100) and (001) are the most common cleavage planes which can be seen from wolframite crystals. According to the mineralogical characters of wolframite, these planes are easy to be formed when cracked by force. Therefore, their surface properties should be related to the absorption of collector ions, and then, to the flotation behavior of wolframite. Actually, wolframite is an isomorphous series with doped iron or manganese atoms between huebnerite (MnWO4 ) and ferbnerite (FeWO4 ). The Mn/Fe ratio of wolframite is variable and there is little evidence of the presence of pure MnWO4 or FeWO4 in the nature.
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Table 1 Experimental and calculating lattice parameters of MnWO4 and FeWO4 . Lattice parameters
Experimental values MnWO4
˚ Lengths (A)
a b c
Angles (◦ )
ˇ
4.8277 (8) [12] 5.761 (1) [12] 4.997 (8) [12] 91.14 [12]
FeWO4 4.73 [12] 5.703 [12] 4.952 [12] 90.00 [12]
That is to say, it is difficult to obtain general rules of micro surface features of wolframite from natural minerals by traditional measuring and testing techniques. The simulation techniques of quantum chemistry based on the density function theory (DFT), however, are gradually developed and utilized for the research of crystal microstructure, which allow the atomic and electronic properties of crystal surface to be understood. And these techniques can also be applied in mineral processing to study, for example, the floatability of minerals [7], band structure and density of states (DOS) [8], absorption of micro molecules on surface [9], highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of bonding atoms [8] and so on. In this work, the atom and electron properties and the stability of (010), (001) and (100) surfaces of MnWO4 and FeWO4 have been analyzed with a DFT calculation in the purpose of studying the features of crystal surface structure and their influence on the absorption of anionic collectors. The screening analysis and laboratory flotation test have been carried out and their productions have been also studied for the relationship between cleavage characteristics and flotation behavior by stereomicroscope. 2. Results and discussion 2.1. Computation details 2.1.1. Lattice and slab geometric optimizations The total energy plane-wave pseudopotential based on DFT was used for performing all calculations with the CASTEP
Geometry optimized values
Error (%)
MnWO4
FeWO4
MnWO4
FeWO4
4.855 5.846 5.043
4.71 5.731 4.972
+0.58 +1.47 +0.92
−0.42 +0.49 +0.40
+0.08
+0.58
91.215
90.52
computer code. The BFGS algorithm was recommended to reach the minimum total energy of the system for its stability and efficiency [10,11] with the functional of GGA-PBE (the generalized gradient approximation of Perdew–Burkee–Ernzerhof). The corresponding bulk lattice parameters of first-principles calculation in bulk geometric optimization as well as the experimental values were displayed in Table 1. The spin polarization was taken into account. The cutoff energy for plane-wave expansion was set to 700 eV and Brillouin-zone integration was performed with the 4 × 4 × 4 and 4 × 4 × 1 k-grid using the Monkhorst–Pack method for bulk optimization and surface structure optimization respectively, which could guarantee acceptable calculation errors. When the total energy changes were converged to less than 1 × 10−5 eV/atom, ˚ the stress less than 0.05 GPa, and the the forceless than 0.03 eV/A, ˚ the relaxation of all atoms was displacement less than 0.001 A, finished. The crystals of MnWO4 and FeWO4 are both monoclinic and belong to the space group P2/C. They have similar lattice parameters according to Table 1. The cleavage planes perpendicular to the three coordinate axes x, y and z are the cleavage planes (100), (010) and (001). When cleaving surfaces along a certain orientation we find the atom distribution of each surface is not unique (see in Fig. 1). What is more, the two newly generated surfaces may be symmetrical or non-symmetrical, which will impact the calculations of surface energies. For calculating the surface energies, the symmetrical surfaces are calculated using one slab while the nonsymmetrical surfaces need two. Each slab contains twelve layers of
Fig. 1. Different slab models of three cleavage planes. The red, blue and purple balls respectively denote oxygen tungsten and manganese (iron) atoms. Slabs of (010)-A, (010)-C and (001)-A have symmetrical surfaces while the others are non-symmetrical. (For interpretation of the references to color in this legend, the reader is referred to the web version of the article.)
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Table 2 Surface energies of different cleavage planes. Plane type
2.1.2. Surface energy calculations For symmetrical slabs, such as (010)-A, the surface energy after relaxation will be simply calculated by Eq. (1) [10,14,15],
Surface energy (J/m) MnWO4
FeWO4
(010)-A (010)-B1 (010)-B2 (010)-C
1.3537
2.5143
5.5908
6.8473
0.6926
0.9186
(001)-A (001)-B1 (001)-B2
1.6228
2.8964
4.4784
6.1233
4.0291
4.9334
4.5262
5.3920
(100)-A1 (100)-A2 (100)-B1 (100)-B2
MnWO4 (average)
FeWO4 (average)
2.5457
3.4267
3.0506
4.5099
4.2777
5.1627
Esur =
Erel-slab − nEbulk 2S
(1)
where Erel-slab is the total energy of relaxed slab, Ebulk is the bulk energy, n means the number of bulks contained in a slab, S denotes the surface area and the number 2 means the two surfaces of a slab.
Fig. 2. Bonding relationship among O, W and Mn(Fe) atoms in three slabs before relaxation. The purple, blue and red balls respectively denote the Mn(Fe), W and O atoms. (For interpretation of the references to color in this legend, the reader is referred to the web version of the article.)
atoms. And the four top layers and four bottom layers are relaxed in the vacuum to form a surface state while the four middle layers are constrained to represent the bulk state and this method was particularly discussed by Mastrikov et al. [13].
Fig. 3. Total charge density distribution (A) and charge density difference distribution (B) of MnWO4 and FeWO4 bulk. The blue areas in B denote the loss of charges while the red areas mean the gain of charges. (For interpretation of the references to color in this legend, the reader is referred to the web version of the article.)
Table 3 Bond lengths and Mulliken populations of bulk and relaxed slabs of MnWO4 and FeWO4 . The bond lengths and populations of (001)-A given in the round bracket indicate the two bonds of the same type have different values, where in (010)-A and (010)-C they have the same values. Bond type
Bulk
(010)-A
˚ Bond length (A)
Mn-O-I Mn-O-II Mn-O-III W-O-I W-O-II W-O-III Fe-O-I Fe-O-II Fe-O-III W-O-I W-O-II W-O-III
In experiment
In models
2.1 [19] 2.16 [19] 2.27 [19] 1.79 [19] 1.92 [19] 2.13 [19] 1.86 [20] 2.16 [20] 2.23 [20] 1.7 [20] 1.97 [20] 2.28 [20]
2.132 2.153 2.304 1.812 1.926 2.155 2.085 2.1 2.101 1.822 1.93 2.131
(010)-C
(001)-A
Population
˚ Bond length (A)
Population
˚ Bond length (A)
Population
˚ Bond length (A)
Population
0.26 0.14 0.13 0.74 0.48 0.3 0.27 0.16 0.19 0.7 0.51 0.28
2.379 2.064 2.444 1.821 1.784 – 2.499 1.944 2.152 1.8405 1.764 –
0.14 0.22 0.09 0.63 0.71 – 0.1 0.27 0.16 0.56 0.73 –
2.043 2.109 – 1.799 1.928 2.191 1.947 2.033 – 1.81 1.9346 2.1996
0.24 0.22 – 0.71 0.44 0.27 0.26 0.26 – 0.7 0.41 0.27
2.171 (2.131) 2.124 2.751 (2.07) 1.78 (1.821) 1.839 2.552 (1.812) 2.078 (2.16) 1.998 2.137 (2.018) 1.78 (1.917) 1.833 2.294 (1.817)
0.25 (0.28) 0.19 0.02 (0.24) 0.71 (0.66) 0.65 0.02 (0.65) 0.27 (0.26) 0.21 0.21 (0.22) 0.71 (0.52) 0.65 0.12 (0.67)
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Fig. 4. Distortions and stretches of bond angles and bond lengths of relaxed slabs. The capital letter A, B and C denote (010)-A, (010)-C and (001)-A of MnWO4 while lowercase letter a, b and c represent (010)-A, (010)-C and (001)-A of FeWO4 , respectively.
For non-symmetrical slabs, such as (010)-B1 and (010)-B2, we calculate the surface energies of both slabs with Eq. (1) respectively, and then average them. In terms of the calculation method above, we obtain the surface energies of different slab models and list them in Table 2. According to Table 2, we can draw the following conclusion. Firstly, all the energies of FeWO4 surfaces are slightly higher than the counterparts of MnWO4 . It means that the FeWO4 surfaces have higher activity than MnWO4 surfaces. Secondly, among three types of crystal surfaces, the (010) has the lowest average surface energy, followed by the (001) and (100). Moreover, the surface energies of (010)-C and (010)-A and (001)-A are prominently lower than others’. It implies that these three kinds of surfaces probably appear more frequently in crystals. Finally, considering the surface atom distribution with surface energy, we find that in the (010) and (100) surfaces the energy of surfaces with manganese (or iron) atoms exposed to the vacuum is lightly smaller than one with tungsten atoms exposed, which means the former is more stable than the latter. In the light of Bond’s breakage theory [16], when cracking, the less the surface energy of new formed surface is, the more stable the surface is. Moreover, as the cracked crystal particle size decreases, the specific surface area grows so quickly that the surface energy becomes the crucial influence on the breakage of fine particles. Thus we can speculate that if the surface energies of some crystal cleavages are prominently lower than others, they will be easily generated. We understand that the surface energies of (010) and (001) planes of wolframite are remarkably lower than one of (100)
plane according to the calculated results above, and each two of the three planes are approximately orthogonal in space. Therefore, the cracked particles will be gradually shaped to be platy to generate as many as possible (010) and (001) planes with the decrease of particle size. This speculation will be further confirmed in Section 2.2. In addition, the surface energies of (010)-A, (010)-C and (001)-A are extremely lower than others. It will be discussed in detail. 2.1.3. Bonding and charge density of surface atoms The bonding and charge density of surface atoms of (010)-A, (010)-C and (001)-A of MnWO4 and FeWO4 are analyzed for understanding their influence on absorption. The bonding relationships of slab surface atoms are mapped in Fig. 2. It can be clearly seen that there are only three sorts of bonding ways among W, O, Mn and Fe atoms, and they are W-O, Mn-O and Fe-O. Each unit area of slab surface has equal numbers of breaking bonds, including four breaking bonds of W-O-III in slab (010)-A, four Mn(Fe)-O-III in (010)-C, two W-O-II and two Mn(Fe)-O-II in (001)-A. A slab surface reconstruction has taken place after relaxation, which leads to new bonding relationship. This can be appropriately depicted by bond lengths and Mulliken populations. In terms of these parameters, the bonding ways among W, O, Mn and Fe can be sorted in six types and they are listed in Table 3. W-O has shorter bond lengths but higher population, compared with Mn(Fe)-O, which reflects that the bond strength of W-O is greatly stronger than that of Mn(Fe)-O. Besides, Fe-O is a little more intensive than Mn-O in bond strength. This trend is in accordance with the elemental
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Fig. 5. Charge density difference distribution of three kinds of slabs after relaxation. A, B and C denote (010)-A, (010)-C and (001)-A of MnWO4 while a, b and c represent (010)-A, (010)-C and (001)-A of FeWO4 , respectively.
electronegativity difference where the W has the highest Pauling electronegativity (2.36), followed by Fe (1.83) and Mn (1.55) [17]. The charge density distribution is quite intuitive to reflect the bonding relationship. In Fig. 3, the total charge density distribution and charge density difference distribution of MnWO4 and FeWO4 bulk are respectively drawn to show the interaction between O and other atoms. From Fig. 3A we can realize that W and O are bonding with a high degree of orbital overlap while the degree of orbital overlap between Mn(Fe) and O is quite low. In addition, the Fig. 3 B shows that an obviously intensive gain and loss of charges gather around the area of electronic orbital overlap between W and O, but that cannot be seen in Mn-O or Fe-O. Considering it with the population and bond length together, we reach the conclusion that the bonding nature between W and O is close to covalent bond while Mn-O and Fe-O are tend to be electrovalent bonds. It can also be seen from Fig. 2 and Table 3 that the W and O atoms exposed to vacuum of (010)-A is unsaturated because of the breaking bond W-O-III. And the bonds between unsaturated O and adjacent W, Mn or Fe differ from the counterparts of bulk. For Mn-O-I or Fe-O-I, the bond lengths increase while the populations decrease, but for W-O-II, the situation is just opposite. It demonstrates that the breaking of W-O-III bond, on one hand, enhances the bond W-O-II, but weakens the bond Mn(Fe)-O-I on the other hand. It contributes to the bond nature among W, O, and Mn(Fe) atoms. Since the bond Mn(Fe)-O-I is electrovalent, the existence of unsaturated O atoms will cause the electrons between O and Mn(Fe) more close to O, that, on the contrary, is just more
beneficial to the formation of covalent bond between O and W. However, in slab (010)-C the breaking bonds are Mn(Fe)-O-III. Though it has a reinforced compensation to Mn(Fe)-O-I and Mn(Fe)O-II, it has little effect on bonding between W and O as the bonding interaction from W-O is extremely stronger than one from Mn(Fe)O. For further study of the bonding relationship of surface atoms of (010)-A, (010)-C and (001)-A, we marked out the bond length and angle changes in relaxed slabs and mapped the charge density difference distribution of relaxed slabs (see Figs. 4 and 5). In (010)-A and (010)-C, all the unsaturated Mn, Fe and W atoms have orbitals to accept electrons from anionic collector, but the spatial configuration and bonding effects of W with adjacent O determines W is hard to be an absorption site. Because of the strong bonding interaction between W and O, the surface W even can combine with adjacent four O atoms to form a stable tetrahedral geometry. Due to the distortion of bond angles and shrink of bond length, the O atoms are spatially higher than and block the W atoms. From Fig. 5B and b we can see the part of dxy of W toward to the vacuum is blocked by the orbitals of O atoms which causes W atoms not to bond with collector ions. On the other side, the Mn and Fe atoms are not blocked by O atoms (see Fig. 5A and a), so their unoccupied orbital dz2 can easily accept electrons to form a bond. However, the situation is a little different in (001)-A as it has two types of breaking bonds W-O and Mn(Fe)-O at the same time and the unoccupied orbital of unsaturated W or Mn(Fe) atoms are not blocked by O atoms (see Fig. 5C and c). That means all of them can be the absorption sites.
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Fig. 6. Photos of screening and flotation products under stereo microscope. A, B and C respectively represent three fractions of −74 + 50 m, −50 + 40 m and −40 m of wolframite raw ores. D and E respectively denote the flotation concentrates and tailings from the size fraction of −50 + 40 m feedings.
2.2. Results of flotation test and stereo microscope analysis of flotation production
directly defined as a ratio of dry weights of the concentrate to the feeding,
To verify our speculation of the relationship between particle size and cleavage plane features of wolframite, we classified a batch of pure minerals of wolframite into three size fractions of −74 + 50 m, −50 + 40 m and −40 m by screening, and observed their cleavage plane characteristics by stereomicroscope. The photos of products from three fractions are given in Fig. 6. It can be clearly seen that most of particles from the fraction of −74 + 50 m are granular that they have irregular cleavages. Then the platy fraction of products rises rapidly while the granular reduce with the decline of size. It is worth noting that the majority of platy particles in grades of −50 + 40 m and −40 m have perfect cleavage planes. This suggests that the occurrence frequency of platy particles as well as the perfect cleavage planes gradually increases with the decline of particle size, and that is in line with the speculation of surface energy calculation results. The fraction of −50 + 40 m was chose for flotation test to study the relationship between flotation behavior and cleavages of wolframite with BHA as collector. The effect of dosage of BHA was also involved. The concentrates and tailings from a flotation test with a dosage of 65 mg/L of BHA were both photographed (see in Fig. 6). From Fig. 6 we can see an overwhelming proportion of platy particles with clearly perfect cleavages in concentrates, which is far more than one in tailings and raw ores. It indicates these particles are easier to be gathered and floated into concentrates by collector. The pH of pulp was 7. The terpenic oil was used as frothing agent (16 mg/L) and BHA was used as single collecting agent. The result from Fig. 7 shows that the floatability of wolframite is quite low without BHA and it gradually grows with the dosage growth of BHA. Since the wolframite was pure, the recovery (ε) is
ε=
˛ × 100%, ˇ
where ε, ˛ and ˇ respectively denote the wolframite recovery, dry weight of concentrate and dry weight of feeding. It is worth mentioning that when the recovery of wolframite reaches to about 50%, it can hardly be boosted even using a high dosage of BHA. And this phenomenon can also be seen in Deng. L’s research [18]. What is more, there are still a small number of platy particles with perfect cleavages left in tailings (see in Fig. 6E). According to the calculation results and the conclusion we have, we can easily come to the explanation of these questions above. The (010) cleavage plane is the most common occurrence of wolframite crystals and it also has 100
Wolframite
Recovery (%)
80
60
40
20
0
0
20
40
60
80
100
Dosage of BHA (mg/L) Fig. 7. Relationship between dosage of BHA and recovery of wolframite.
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Fig. 8. Atomic bonding relationships and charge density distribution of BHA absorption models on (010)-A, (010)-C and (001)-A planes before and after geometry optimization.
two types, the (010)-A and (010)-C. If the wolframite particles with cleavages mainly dominated by (010)-A which exposing W atoms without Mn or Fe atom, they can hardly be absorbed by BHA. As it is almost impossible to forbid the random and accessible occurrence of (010)-A in wolframite particles, a part of particles without absorption of BHA are left in tailings, which leads to a low recovery of wolframite. 2.3. Absorption of BHA on different surfaces To further understanding the absorption of BHA on different mineral surfaces, a series of simulations based on the models above were performed. Considering the similarities between MnWO4 and FeWO4 surfaces, only the absorption on (010)-A, (010)-C and (001)A planes of MnWO4 was simulated. The red, dark blue, gray, purple and light blue balls respectively denote O, N, C, Mn and W atoms. A, B and C respectively depict absorption of BHA on (010)-A, (010)-C and (001)-A planes of MnWO4 , while a, b, and c respectively represent their corresponding charge density distribution. Fig. 8 has recorded the main atomic changes of absorption of BHA on different planes before and after geometry optimization and mapped their charge density distribution. We can clearly see obvious absorption occurring on different planes except of (010)A. Fig. 8-A shows that there is no bonding between exposed W atoms of (010)-A and O atoms of BHA as they have never reached to
the effective bonding range (according to Table 3 the bond lengths ˚ and showing no electron density would never be longer than 3 A), enhancement around W and O atoms, Fig. 8-a goes a step further to demonstrate it. On (010)-C plane, the distances between Mn and two O atoms are reduced respectively from 3.128 A˚ and 3.168 A˚ ˚ while on (001)-A plane, from 3.697 A˚ and to 1.970 A˚ and 2.041 A, ˚ And the electron density distribu3.728 A˚ to 2.109 A˚ and 2.298 A. tion of bonding atoms has overlapped. It is worth to note that the absorption on (001)-A plane is based on the bonding among one Mn atom, one W atom and two O atoms, accompanied with a breakage of one inner Mn-O bond (the Mn-O bond has been stretched from 2.134 A˚ to 3.143 A˚ which is longer than effective bonding distance of Mn-O). All these calculating results demonstrate that BHA can absorb on (010)-C and (001)-A planes but not on (010)-A planes. It agrees well with our explanation for the results of pure mineral flotation test in Section 2.2 and strongly support our viewpoints. 3. Conclusion In this study a DFT calculation was carried out to obtain the surface energies, bonding nature, Mulliken population and charge density distribution of the (010), (001) and (100) planes of MnWO4 and FeWO4 , and the cleavage plane characteristics of screening and flotation production were analyzed by stereo microscope. Based on the results of this study, we conclude the following points:
X.Y. Qiu et al. / Applied Surface Science 367 (2016) 354–361
(1) According to the calculation result of surface energy, the (010) plane is the easiest one to be seen in wolframite, followed by (001) plane and (100) plane. (2) The bonding nature and spatial distribution of surface atoms demonstrate that (010) and (001) are the primary planes for the absorption of anion collector ions, in (010) plane of which only Mn and Fe can be the absorption sites while in (001) plane W can also be the absorption site. (3) The stereo microscope observations of screening and flotation productions show that the cleavage plane features of wolframite particles are in accordance with the prediction based on calculation results, namely, the proportion of platy particles along with their perfect cleavage planes increase with the decrease of particle size. (4) When using BHA alone as the collector of wolframite, the recovery of wolframite around 50% can hardly be further boosted even using a high dosage of BHA, that is mainly because a part of cleavage planes of wolframite is occupied by W atoms and hard to be absorbed by collector ions. Acknowledgements This research is supported by High Performance Computing Center of Central South University of China. The authors wish to thank them very much. References [1] Pradip, Recent advances in the recovery of tungsten values in the fine and ultrafine size range, Bull. Mater. Sci. 19 (2) (1996) 267–293. [2] Y. Hu, D. Wang, Z. Xu, A study of interactions and flotation of wolframite with octyl hydroxamate, Miner. Eng. 10 (6) (1997) 623–633. [3] D. Wang, Y. Hu, W. Hu, Flotation behavior of wolframite with different components, J. Central South Inst. Min. Metall. (China) (4) (1986) 40–45. [4] W. Dawei, W. Kewu, Q. Jicun, Hydrophobic agglomeration and spherical agglomeration of wolframite fines, Int. J. Miner. Process. 17 (3) (1986) 261–271.
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