Comment on: Effects of Crystal Chemistry on Sodium Oleate Adsorption on Fluorite Surface Investigated by Molecular Dynamics Simulation: Renji Zheng, Zijie Ren, Huimin Gao, Zhijie Chen, Yupeng Qian, Yubiao Li, Minerals Engineering, Vol. 124, Pp. 77–85, 2018

Yann Foucaud, Michael Badawi, Lev Filippov

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Yann Foucaud, Michael Badawi, Lev Filippov. Comment on: Effects of Crystal Chemistry on Sodium Oleate Adsorption on Fluorite Surface Investigated by Molecular Dynamics Simulation: Renji Zheng, Zijie Ren, Huimin Gao, Zhijie Chen, Yupeng Qian, Yubiao Li, Minerals Engineering, Vol. 124, Pp. 77–85, 2018. Minerals Engineering, 2019, 135, pp.156-159. 10.1016/j.mineng.2019.02.020. hal-03603340

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Comment on: Effects of crystal chemistry on sodium oleate adsorption on fluorite surface investigated by molecular dynamics simulation: Renji Zheng, Zijie Ren, Huimin Gao, Zhijie Chen, Yupeng Qian, Yubiao Li, Minerals Engineering, vol. 124, pp. 77-85, 2018.

Dear Sirs,

In a recent paper, (Zheng et al., 2018) discussed new approaches that link the crystallographic properties of fluorite with its behaviour in flotation. They suggested that an anisotropy is exhibited when the size of mineral particles is reduced, resulting in different surfaces generated. The differences between the behaviour of various size fractions of fluorite could then be attributed to the differences in the surface properties. This approach is crucial to gain understanding in the floatability of fine particles as fine-grained mineralization deposits will be predominant in the next decades. However, the authors made several statements based on unclear or sometimes wrong data or interpretation. Herein we discuss all the points deserving discussion or clarification.

## Crystallographic properties

The Table 2 should be revised. Indeed, according to (Gao et al., 2012), the surface energy displays a linear correlation with the density of broken surface bonds ($D_b$). However, the authors, in the Table 2, present results that are incoherent with the previous statement: the surface energies and the density of broken surface bonds are not correlated at all. The surface energies presented in the Table 2 seem correct, calculated from first principles methods and in accordance with the fact that the (111) is the most exposed surface by the natural cleavage (Bennewitz et al., 1997; De Leeuw et al., 2000; Gao et al., 2012; Maldonado et al., 2013; Parks and Barker, 1977; Puchin et al., 2001). The observed incoherence can be attributed to the number of broken bonds per unit cell area, which is not 4 but 2 for the (111) surface, described below (Fig. 1) and by (Gao et al., 2012). This mistake can be explained by the fact that the authors generated the wrong (111) surface. Indeed, the surface displayed in Fig.5 and used for MD simulations in Fig.6 is not the most favoured. According to (De Leeuw et al., 2000; Foucaud et al., 2018; Gao et al., 2012; Maldonado et al., 2013; Puchin et al., 2001), the most favoured (111) surface is composed of a plane of calcium atoms with one fluorine atoms layer a bit above and another layer of fluorine atoms a bit below (Fig. 1). For the (111) surface displayed by the authors, the fluorine atoms plane above the calcium atoms plane is missing. It induces that the calcium atoms are four-coordinated instead of being seven-coordinated as it should be the case. Also, it means that, for each unit cell area, four bonds are broken instead of only two. Furthermore, on the two broken bonds admitted by each unit cell area for the (111) surface, one is a broken F-Ca bond and the other is a broken Ca-F bond. The $D_{b(Ca-F)}/D_b$ should then be 50% for the (111) surface and not 100% as it is mentioned in Table 2. It is exactly the same than for the (110) surface, which exhibits four broken bonds: two broken F-Ca bonds and two broken Ca-F bonds.
<figure><img src="image_2.png" /><figcaption>Fig. 1. Fluorite bulk cut following the red plane corresponding to the (111) cleavage plane, generating the most favoured (111) surface. Blue: Ca atoms; Grey: F atoms; Red dashed line: broken Ca-F bond; Green dashed line: broken F-Ca bond; Black plane: unit cell area.</figcaption></figure>

All the broken bonds are completely the same and it is really unclear to speak about “Ca-F” and “F-Ca” bonds. Indeed, the Ca atoms have not unsaturated bonds: broken bonds are not compensated by the closest atoms in terms of electrons. Hence, the atoms on the surface are not neutral, which explains the existence of a global surface energy that authors calculate in the paper. Also, the crushing stage leads to the generation of two surfaces that, gathered, should reform the bulk. For the (311) surface, the authors report 5 Ca-F broken bonds out of 6 total broken bonds. It indicates that the reciprocal formed surface has 5 F-Ca broken bonds to respect the global stoichiometry of the uncrushed particle. Thus, if the fluorite is cleaved on the (311) surface, it will form a mixture of Ca-terminated surfaces and F-terminated surfaces in equal proportion. A calculation on the number of calcium atoms on the surface is then completely false because the F-terminated surfaces are not taken into account. The authors should have generated both the Ca-terminated and the F-terminated surfaces to compare the adsorption of oleate and water onto them. As the adsorption depends mainly on the calcium atoms, we do think that the response of the surface-water-oleate system will be totally different between the Ca-terminated and the F-terminated surfaces. The main aim of the paper is to link crystallographic properties to macroscopic phenomena as flotation: the global response of the (311) surface should be studied and not only the half (Ca-terminated surfaces). The same remark can be done for the (111) surface: the authors display a surface where the uppermost atoms are calcium. This surface can be called a Ca-terminated one. The reciprocal surface generated with this method should then be terminated by two planes of fluorine atoms, as displayed in Fig. 2

<figure><img src="image_3.png" /><figcaption>Fig. 2. Fluorite bulk cut following the red plane corresponding to the (111) cleavage plane, generating a Ca-terminated and a F-terminated (111) surfaces. Blue: Ca atoms; Grey: F atoms; Red dashed line: broken Ca-F bond; Green dashed line: broken F-Ca bond; Black plane: unit cell area.</figcaption></figure>

It induces that, during the crushing stage, the two surfaces are formed in equal proportion and then the F-terminated surface should also be studied. However, in such case, the calcium atoms are eight-coordinated, resulting in no reactivity for them, while the surface fluorine atoms are just two-
coordinated which seems very unfavoured. At least, the comparison between the different methods that can be used to generate the surfaces should be compared through a complete DFT study.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Fluorite bulk cut following the red plane corresponding to the (311) cleavage plane, generating the most favoured (311) surface. Blue: Ca atoms; Grey: F atoms; Red dashed line: broken Ca-F bond; Green dashed line: broken F-Ca bond; Black plane: unit cell area.</figcaption></figure>

We think that only one type of the (311) surface exists and that the one generated by the authors is wrong. We do not think that 5 Ca-F bonds are broken. Indeed, if it was the case, it would let a four-coordinated calcium on the surface, which seems really unfavoured in terms of energy. The high surface energy for this surface, presented in Table 2, could be attributed to this wrong input configuration of the surface. We rather think that the (311) surface is the one presented in our Fig. 3, where 3 Ca-F and 3 F-Ca bonds are broken, resulting in two equal reciprocal surfaces. It also means that five-coordinated calcium atoms are let on the surface instead of four-coordinated calcium atoms as it is presented by the authors. However, this case should be studied through a full DFT study, to compare the different configurations and to determine which one is preferred for the (311) surface, as it is usually done when surfaces are generated in DFT studies (Gao et al., 2012; Hu et al., 2012). Nevertheless, it can be assumed that the configuration with four-coordinated calcium atoms will have a higher surface energy than the configuration with five-coordinated calcium atoms. The most favoured surface should be considered for this study as the link with flotation properties is done. Since the authors do not consider the real surface, no discussion can be made between crystallographic properties and flotation responses. We would want to remind the authors that surfaces cannot be generated anyhow and the crystallographic principles should be applied as they are based on the minimization of the energy inside the crystal, occurring naturally during the crushing phase. Also, the surface energies displayed in Table 2 are wrong for the (111) and the (311) surfaces as they are not the most favoured ones (and then the naturally occurring ones).

## Flotation results

It appears difficult to discuss the flotation results displayed in Fig. 4. First, the flotation performed on a non-deslimed material (0-45 µm) cannot be compared with other size fractions because the fine particles are known to be mechanically entrained and to lead to high collector consumption. Also, the differences in floatability can be just explained by the variation of specific surface area: much more collector is needed to make hydrophobic the fluorite surface for fine particles. The hydrophobic/hydrophilic balance may be less favourable for the flotation of finer size fraction even if the ratio between exposed cleavage planes is the same. Besides the higher mechanical entrainment for the 0-45 µm size fraction, a better flocculation ability has also to be considered for this fraction.

Furthermore, the solubility rate of the mineral is kinetically enhanced as the specific surface area is higher. The fluorite is a semi-soluble salt and this higher dissolution rate induces a worse adsorption of the collector. The gravity force should also be considered as it depends on the particle size and is
included in the forces balance that controls the attachment-detachment probabilities ($P_A$, $P_D$, respectively). Then, no conclusion can be done on the floatability graphs as no correction has been done on them to take into account the specific surface, the mechanical entrainment and the flocculation ability.

Furthermore, the UV-vis absorption spectra are not in accordance with the interpretations done on the floatability graphs. Indeed, the three different spectra (S1, S2, S3) display very different behaviours for the three size fractions while the three different floatability graphs (S1, S2, S3) demonstrate very close behaviours for the three size fractions. On each spectrum, the absorption rate repartition between the size fractions is different, indicating that no conclusion can be done on them. Finally, no conclusion at all can be done since, when the Table 2 will be corrected, there will be no difference between the (111) and the (110) surfaces in terms of calcium sites density.

## Molecular dynamics simulations

The energies computed by molecular dynamics simulation using a COMPASS force field in Table 3 are aberrant and present huge heterogeneity between the different surfaces, from -25 313.08 kcal/mol for the (111) surface to -223.60 kcal/mol to the (110) surface. It could derive from the non-adsorption of oleate molecules on the (110), showed on Fig. 6, b. In that case, interaction energies cannot be presented as the oleate molecules just interact between themselves and not with the surface. In addition, it is well admitted that a classical chemisorption displays adsorption energies between -100 and -500 kJ/mol, corresponding to -25 to -125 kcal/mol, 1000 times lower than some of the adsorption energies presented in Table 3. Moreover, if we assume that the author describe physisorptions, these latter range from -10 to -100 kJ/mol, corresponding to -2.5 to -25 kcal/mol. Are the unities correct for Table 3? These aberrant adsorption energies could be attributed to highly reactive (111) and (311) surfaces as we have shown previously that the surfaces used in this work are probably not the most energetically favoured. Indeed, the four-coordinated calcium atoms are highly reactive compared to the seven-coordinated ones. The study should be conducted again with the correct surfaces. The only correctly generated surface in this work, namely the (110) one, displays no adsorption of the oleate onto it. The only adsorptions reported during this work are artefacts made of electrostatic adsorptions permitted by high reactive calcium atoms due to crucial mistakes during the surface generation.

For the MD simulations, it is very weird for us to observe a well-organized structure of the water bulk but only before the MD simulation. The snapshots presented after the MD simulations in Fig. 6 are very strange for the (111) and the (311) surfaces. After the MD simulation, for these surfaces, all the water molecules are oriented towards the top of the cell, which appears very weird considering the clustering of water molecules described in the literature (Anacker and Friedrich, 2014; Foucaud et al., 2018; Gillan et al., 2016; Liu et al., 1996; Ludwig, 2001; Maheshwary et al., 2001; Møgelhøj et al., 2011; Perera, 2011; Roy et al., 2005; Skinner et al., 2014). Besides, it appears very difficult for oleate molecules to destroy the water structure in the bulk at several nanometers far and this orientation cannot be attributed to the oleate molecules.

The adsorption configuration of the oleate molecules cannot be discussed for the presented (311) and (111) surfaces as they are not the most energetically favoured. Four-coordinated calcium are highly reactive on the surface and may differ a lot from five-coordinated or seven-coordinated calcium atoms, respectively. Moreover, the Fig. 7 presents carboxyl groups with a double-bonded oxygen and a single-bonded oxygen in anionic form. When the deprotonation reaction occurs, the lone electron is delocalized on the whole carboxyl group and the two C-O bonds are perfectly equivalent, being partly double.

The relative concentration of carbon and oxygen in NaOl is a weird indicator for monitoring the distance between oleate molecules and the surface. What does it mean? We can imagine that it
displays the distances between the surface and the oleate. We are wondering what point is chosen as the reference point for the surface. Moreover, the distance displayed is not in accordance with what is described on Fig. 7, as the authors mention around 2 Å between the oxygen atoms and the surface calcium atoms.

## X-Ray Diffraction analyses

The fluorite is not an anisotropic mineral but an isotropic mineral. It is part of the cubic crystallographic system which induces that $a = b = c$ and $\alpha = \beta = \gamma = 90^{\circ}$. It means that all the fluorite properties are exactly the same in the three spatial directions, which is the definition for “isotropic mineral”. However, the isotropy is not considered for the cleavage of the mineral. The authors do not explain the reason why the fluorite could develop cleavages along the (110) plane when the particle size is decreased. Also, the fact that the (111) plane is the characteristic diffraction plane for the fluorite is due to the high atom number in the structure factor, indicating that the (111) plane contains many atoms. Then, it does not necessarily point out that it is the main cleavage plane. The X-Ray Diffraction is a bulk analysis method and surfaces can be hardly analysed with this method. First, the authors do not precise what procedure has been followed for the XRD analyses. The method used for peaks comparison is not well explained and we do not think that it is correct. Indeed, the full width at half maximum should be used instead of the surface area under the peak. We do think that the observations on the peak comparison on XRD patterns are pure artefacts. When the fluorite particles size is increased, the particles orientate easily, forming large surfaces that can set on the XRD medium. The XRD pattern from JCPDS display a $I_{(220)}/I_{(111)}$ much more higher than the 0-45 µm pattern. It can be explained by the very small size of the fluorite particles analysed by XRD for the establishment of JCPDS files. It is well known than the particle size impact strongly the XRD pattern and mostly the relative intensities of the peaks. Indeed, the peak intensity is correlated with the preferential orientation term that describes this artefact. It induces that the XRD analyses should always be performed on very fine product (< 20 µm) to avoid the particle effect on the patterns. The relative peak intensities are not a function of the exposed surface but can be sensible to the shape factor. This term takes into account the interference of the waves diffracted by the different crystalline cells and can be high when the crystallite size becomes very low (< 1 µm). However, the induced enlargement is observed for all the peaks and for size fractions that are much finer than the one studied by the authors. Following the basic principles of the crystallography, the cleavage plane should stay the same no matter the size of the particles. However, we can easily suggest than when the primitive cell is approached in terms of size, the main cleavage plane cannot be still applicable due to the low atom number remaining in the particle. The primitive cell of the fluorite is approximatively 5 Å, which is 10 000 times lower than the size of the particle in the present study. It is then weird than, at this kind of size, the (111) cleavage plane becomes no more dominant in the fluorite, above all between the 45-74 µm size fraction and the 74-100 µm, which can be considered as the same with a crystallographic point of view.

## Conclusions

In sum, the change from the (111) cleavage plane to the (110) cleavage plane when the size fraction is reduced should be considered as a simple hypothesis since no strong scientific proof is obtained from experimental data. Morphometric analyses should be conducted to prove the hypothesis but not with XRD, which is a bulk method strongly dependent on the size fraction and then impacted by an artefact. The floatability studies are incorrectly exploited and cannot prove any morphology change in the fluorite particles when their size is decreased, as many parameters impacting the flotation recovery have not been taken into account. The UV-vis absorption spectra do not bring any clue as they display complete different behaviours for the three fluorites that, yet, seem very similar in terms of flotation behaviours and chemical analyses. Finally, the molecular dynamics simulations are based on wrong non-stoichiometric surfaces that are highly reactive as they are not the most energetically favoured. The computed interaction energies are of several range of magnitude higher than those expected in this
kind of system. Moreover, the orientation of the water molecules in the cell presented fully contradicts
the results of the literature showing water clusters.

References

Anacker, T., Friedrich, J., 2014. New accurate benchmark energies for large water clusters: DFT is better than expected. Journal of Computational Chemistry 35, 634–643.
https://doi.org/10.1002/jcc.23539

Bennewitz, R., Reichling, M., Matthias, E., 1997. Force microscopy of cleaved and electron-irradiated CaF2(111) surfaces in ultra-high vacuum. Surface Science 387, 69–77.
https://doi.org/10.1016/S0039-6028(97)00268-9

De Leeuw, N.H., Purton, J. a A., Parker, S.C., Watson, G.W., Kresse, G., 2000. Density functional theory calculations of adsorption of water at calcium oxide and calcium fluoride surfaces. Surface science 452, 9–19.

Foucaud, Y., Badawi, M., Filippov, L.O., Filippova, I.V., Lebègue, S., 2018. Surface Properties of Fluorite in Presence of Water: An Atomistic Investigation. The Journal of Physical Chemistry B 122, 6829–6836. https://doi.org/10.1021/acs.jpcb.8b02717

Gao, Z., Sun, W., Hu, Y., Liu, X., 2012. Anisotropic surface broken bond properties and wettability of calcite and fluorite crystals. Transactions of Nonferrous Metals Society of China 22, 1203–1208. https://doi.org/10.1016/S1003-6326(11)61306-X

Gillan, M.J., Alfè, D., Michaelides, A., 2016. Perspective: How good is DFT for water? The Journal of Chemical Physics 144, 130901. https://doi.org/10.1063/1.4944633

Hu, Y., Gao, Z., Sun, W., Liu, X., 2012. Anisotropic surface energies and adsorption behaviors of scheelite crystal. Colloids and Surfaces A: Physicochemical and Engineering Aspects 415, 439–448. https://doi.org/10.1016/j.col surfa.2012.09.038

Liu, K., Cruzan, J.D., Saykally, R.J., 1996. Water Clusters. Science 271, 929–933.
https://doi.org/10.1126/science.271.5251.929

Ludwig, R., 2001. Water: From Clusters to the Bulk. Angewandte Chemie International Edition 40, 1808–1827. https://doi.org/10.1002/1521-3773(20010518)40:10<1808>::AID-ANIE1808>3.0.CO;2-1

Maheshwary, S., Patel, N., Sathyamurthy, N., Kulkarni, A.D., Gadre, S.R., 2001. Structure and Stability of Water Clusters (H₂O)ₙ, n = 8–20: An Ab Initio Investigation. The Journal of Physical Chemistry A 105, 10525–10537. https://doi.org/10.1021/jp013141b

Maldonado, P., Godinho, J.R.A., Evins, L.Z., Oppeneer, P.M., 2013. Ab Initio Prediction of Surface Stability of Fluorite Materials and Experimental Verification. The Journal of Physical Chemistry C 117, 6639–6650. https://doi.org/10.1021/jp312645f

Møgelhøj, A., Kelkkanen, A.K., Wikfeldt, K.T., Schiøtz, J., Mortensen, J.J., Pettersson, L.G., Lundqvist, B.I., Jacobsen, K.W., Nilsson, A., Nørskov, J.K., 2011. Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like. The Journal of Physical Chemistry B 115, 14149–14160.

Parks, T.C., Barker, W.W., 1977. The ordered dispersal of point defects over cubic lattices: Application to fluorite-related structures. Journal of Solid State Chemistry 20, 397–407.
https://doi.org/10.1016/0022-4596(77)90177-3

Perera, A., 2011. On the microscopic structure of liquid water. Molecular Physics 109, 2433–2441.
https://doi.org/10.1080/00268976.2011.617712

Puchin, V.E., Puchina, A.V., Huisinga, M., Reichling, M., 2001. Theoretical modelling of steps on the CaF2 (111) surface. Journal of Physics: Condensed Matter 13, 2081.

Roy, R., Tiller, W.A., Bell, I., Hoover, M.R., 2005. The Structure Of Liquid Water; Novel Insights From Materials Research; Potential Relevance To Homeopathy. Materials Research Innovations 9, 98–103. https://doi.org/10.1080/14328917.2005.11784911

Skinner, L.B., Benmore, C.J., Neuefeind, J.C., Parise, J.B., 2014. The structure of water around the compressibility minimum. The Journal of Chemical Physics 141, 214507.
https://doi.org/10.1063/1.4902412
Zheng, R., Ren, Z., Gao, H., Chen, Z., Qian, Y., Li, Y., 2018. Effects of crystal chemistry on sodium oleate adsorption on fluorite surface investigated by molecular dynamics simulation. Minerals Engineering 124, 77–85. https://doi.org/10.1016/j.mineng.2018.05.017

Yours sincerely,

Yann Foucaud¹, Michaël Badawi², Lev Filippov¹

¹ Université de Lorraine, Laboratoire GeoRessources, UMR 7359 – CNRS, 2 rue du Doyen Marcel Roubault, 54 505 Vandœuvre-lès-Nancy-Cedex, France.

E-mails: yann.foucaud@univ-lorraine.fr;

² Université de Lorraine, Laboratoire Physique et Chimie Théoriques, UMR 7019 – CNRS, BP239, Boulevard des Aiguillettes, 54 506 Vandoeuvre-lès-Nancy-Cedex, France.

E-mail: michael.badawi@univ-lorraine.fr

Corresponding author : lev.filippov@univ-lorraine.fr