Chunyun Zhu ,Guosheng Li ,Yowen Xing *,Xihui Gui

a Chinese National Engineering Research Center of Coal Preparation and Purification,China University of Mining and Technology,Xuzhou 221116,China

b School of Chemical Engineering and Technology,China University of Mining and Technology,Xuzhou 221116,China

c School of Chemical Engineering and Technology,Zhengzhou University,Zhengzhou 450052,China

Keywords:Adhesion force Water/oil droplet Air bubble Roughness Floatability

ABSTRACT Surface roughness plays a significant role in floatability of coal.In the present paper,coking coal surface was polished by three different sandpapers and the surface properties were characterized by contact angle and roughness measurements.The effect of surface roughness on floatability was investigated by adhesion force measurement system for measuring interaction forces between droplets/bubbles and coking coal surfaces with different roughness.The results showed that the contact angle decreased with increasing roughness yet the adhesion force between the water droplet and coal surface increased owing to the increased contact line and the appearance of line pinning.Maximum adhesion forces between water and surfaces were 111.70,125.48,and 136.42 μN when the roughness was 0.23,0.98,and 2.79 μm,respectively.In contrast,under a liquid environment,the adhesion forces between air bubble/oil droplet and coal surfaces were decreased with increasing roughness because of the restriction by water.Maximum adhesion forces of increasing roughness were 97.14,42.76,and 17.86 μN measured at interfaces between air bubble and coal surfaces and 169.48,145.84,and 121.02 μN between oil droplet and surfaces,respectively.Decreasing roughness could be beneficial to the spreading of oil droplets and the adhesion of bubbles which is conducive to flotation separation.

1.Introduction

Froth flotation is a physicochemical separation process that involves gas,liquid,and solid;it is performed to separate particles by the difference in floatability between the target mineral and gangue[1–3].Floatability,the difficulty in adhesion between bubble and particles,is an important factor that determines the efficiency of flotation.The nature or induced hydrophobicity of the mineral surface directly determines the adhesion strength between particles and bubbles [4–7].

Contact angle measurement has been acknowledged as the most typical method to characterize mineral hydrophobicity,and a larger contact angle represents better floatability[8–10].For coal,its contact angle is considered to be a function of the surface composition and roughness because of its heterogeneity[11,12].In the literature,many methods have been proposed to investigate the effect of surface roughness,such as contact angle measurement[13–16],induction time measurement [17–22],high-speed dynamic visualization [23,24],and adhesion/detachment force measurement [25–27].Chen et al.[28] performed contact angle and induction time measurements to investigate the effect of roughness on coal wettability and discovered that both the attachment time and contact angle increased with decreasing roughness.Xing et al.[29]proposed decreasing the induction time between a bubble and coal surface by decreasing the surface roughness and suggested hypobaric pretreatment to reduce the adverse effects of increasing roughness on induction time.Vaziri Hassas et al.[30] investigated the effect of roughness on glass bead floatability via high-speed dynamic visualization to obtain the attachment efficiency and discovered that increasing the roughness improved the contact angle,bubble adhesion,and flotation separation.In general,the adhesion/detachment force measurement could characterize the floatability and the higher forces represent the better floatability [31–33].Moghadam and Vahedi Tafreshi [34] selected a deionized water droplet containing 15% glycerol to investigate the adhesion forces between a liquid bridge and two parallel plates.They found that during compression-stretching process,the force-spacing trajectory of a liquid bridge on a surface with contact angle hysteresis followed angle-constant and lineconstant lines.Xing et al.[35] applied a home-developed system for measuring the bubble-particle detachment force to investigate the effect of surface roughness on the detachment between bubbles and glass beads with different contact angles.They reported that when the local contact angle of the particle surface was<90°,the detachment force was higher for smooth surfaces.

Many methods rely on image analysis and probability statistics,where the roughness effect is not fully considered[36].Meanwhile,in attachment/detachment force measurements,mineral particles with different roughnesses are typically replaced by glass beads[37].Furthermore,wetting [38,39],adsorption of reagents on the surface [40,41],interaction of inter-particle,and interaction between bubbles and particles [42–44] constitute the basic subprocesses of separation and affect the flotation efficiency.The adhesion can be divided into two types,including normal adhesion and lateral adhesion.The normal adhesion acts on surface in a perpendicular direction against the detachment and the lateral adhesion acts on surface in tangential direction,being related to sliding or depinning [45].In this study,an adhesion force measurement system was adopted to investigate the effect of roughness on the normal interaction force between coal surface and water droplets,air bubbles,and oil droplets.This study is expected to provide fundamental understanding regarding the effect of roughness on floatability.

2.Materials and methods

2.1.Materials

The coking coal used in this study with 7.71% ash content was obtained from a coal plant in Xinjiang,China.The sample was cut into a regular lump with a flat surface using a cutting machine and polished with sandpaper of different roughness (4000,400,and 240 mesh)using a polishing machine.After polishing,the coal surface was washed with a significant amount of deionized water to remove the residual powder.Next,the sample was sealed and stored for subsequent tests.The reagent used in this study,i.e.diesel oil,was purchased from Sinopharm Chemical Co.,Ltd.,China.

2.2.X-ray photoelectron spectroscopy (XPS) measurements

XPS instrument (ESCALAB 250Xi,Thermo Scientific,Waltham,MA,U.S.) was adopted to analyze the types of oxygen-containing functional groups and their relative contents in the coking coal samples.The result was analyzed using the Casa XPS software.The oxygen-containing groups were mainly analyzed because of their significant effect on the floatability of the coal.The binding energy was corrected using the C1s peak at 284.6 eV.

2.3.Roughness measurements

A digital camera was selected to image the coal surface to preliminarily compare its surface roughness.A roughness tester(Mitutoyo SJ-210) was selected to characterize the roughness of the coal surface quantitatively,and the average roughness (Ra)was selected to characterize the roughness and is defined through Eq.(1).

whereNis the total number of points in the measurement line;Zithe height at thei-th point;andthe average height of the entire measurement line.The sensor was in contact with the surface in different directions,and the tests were repeated five times at different locations to minimize the error in the measurement.

2.4.Contact angle measurements

The sessile drop method was selected to measure the contact angle of the surface using a drop shape analyzer (DSA100,KRÜSS,Germany).The surface was kept stable,and then 8 μL water droplet was generated and captured at the capillary port above the surface.The surface was controlled such that it approached the droplet slowly,and an image was captured after 1 s of contact to measure the contact angle.The contact angles were obtained by repeating the measurements five times.

2.5.Adhesion force measurements

A high-sensitivity microelectronic mechanical balance system(JK99M2,China)matched with a digital camera was used to record the adhesion forces between the droplets and the coal surface with different roughness.A schematic diagram of the experimental system is shown in Fig.1.

A 3 μL water droplet/bubble was suspended on the capillary port,which was connected to the microbalance,and the force was set to zero at the initial point of motion.It was difficult to capture the oil droplets on the capillary port directly because their density was lower than that of water.A glass bead with high hydrophobicity adhered to the capillary port,and 3 μL of oil adhered on the glass bead to represent the oil droplet (Fig.1).The cell was filled with water when the bubble/oil droplet was captured on the port.The coal fixed at the bottom of the cell on the mobile stage moved upward at a velocity of 0.01 mm/s.When the droplet/bubble was in contact with the coal surface,an instantaneous adhesion force appeared on the water droplet,unlikely for the oil droplets and air bubbles.Subsequently,the coal continued moving up by 0.3 mm,ensuring the repeatability of the force measurements,and the mobile stage carrying the coal sample withdrew to the initial position at the same velocity.During the retreat,the maximum adhesion forces were recorded,and the pull-off forces appeared after a capillary bridge was formed.Finally,the movement stopped,and the measurements were halted.The adhesion force measurements were repeated five times to minimize the error.

Fig.1.Schematic of the adhesion force measurement system.

Fig.2.XPS wide energy spectra of the coal sample.

3.Results and discussion

3.1.Surface characterizations

3.1.1.Surface chemical composition

The chemical elements in the coal were characterized by XPS,which is a surface-analysis method.The wide spectrum of coking coal is shown in Fig.2,and the semi-quantitative results are listed in Table 1.The carbon content was 77.16%,which significantly exceeded the oxygen content of 17.47%.The low oxygen content indicated that the metamorphic degree of the coal was relatively high with favorable for flotation.C1s peak fitting was performed for further analysis to obtain the chemical groups (Fig.3).As shown in Fig.3,the binding energy of C-C/C-H,C-O,C=O,and O-C=O corresponded to 284.6,285.6,286.6,and 289.1 eV,respectively.The content of C-C/C-H,C-O,C=O,and O-C=O were 62.27%,24.04%,7.05%,and 1.64%,respectively.The content of oxygen-containing groups,such as C-O,C=O,and O-C=O,was relatively low.The floatability of the coking coal was well,and thus,the traditional reagent,diesel oil,was the proper collector for this coal.

Table 1 Semi-quantitative results of the surface chemical composition of the coal sample.

3.1.2.Surface topography

The morphology of the coal surface treated with sandpaper of different meshes is shown in the left panel of Fig.4.As shown,the coal surface after being treated with 400-mesh sandpaper was smooth,with a few scratches.When it was polished using rougher sandpapers,the coal surface presented more scratches and the ravines were deeper.To quantitatively analyze the roughness of the surfaces,roughness measurements were conducted,and the results are shown in the right panel of Fig.4.The law that the surface roughness increases with the decrease in the sandpaper mesh is consistent with the results from morphology observation.The roughness of the coal surface was 0.23,0.98,and 2.79 μm after it was treated using sandpapers of 4000,400,and 240 mesh,respectively.

Fig.3.XPS C1s peaks of coking coal sample.

Fig.4.Coal surface morphology (left) and roughness (right) polished by different sandpaper.

3.1.3.Wettability

Fig.5 shows the relationship between roughness and contact angle using the sessile drop method.The contact angle of the coal surface was 66.0° when its surface roughness was 2.79 μm,and 75.3°for 0.98 μm.After polishing using the 4000-mesh sandpaper,the contact angle increased to 82.5°.The results show that the contact angle of the smoother surface was higher than that of the rougher surface,indicating that using the appropriate polish is beneficial for improving the floatability of coal.

Roughness,which is related to changes in the physical structure of the coal surface,can affect the surface wettability.According to the Wenzel model [46],the grooves on the hydrophilic surface would be filled with water,and hence,the actual solid–liquid interface would increase,resulting in a lower contact angle.The actual contact angle (θa) is related to the Young-Laplace contact angle (θYL),which is expressed as

Fig.5.Effect of roughness on contact angle.

whereris the roughness ratio of the actual surface area to the projected area and is defined as

whereLis the length of contact line inx-direction;and ζsthe wetting perimeter [47].As the mesh of the sandpaper increases,the roughness of the surface decreases andrapproaches 1.For a hydrophilic surface (θYL<90°),θaincreases with decreasing roughness.The results of the contact angle measurement are consistent with the Wenzel theory.This model explains the wettability of surfaces with various roughnesses;however,in the flotation,not only water but also oil and bubble interacting with the coal surface are related to the floatability of coal.To investigate the interaction mechanism of water,bubbles,and oil on the coal surface,force measurements were performed to further understand the effect of roughness on floatability.

3.2.Adhesion behavior between droplets/bubble and coal surface

3.2.1.Water droplet on coal surface

Fig.6 shows the force curves of water during interactions with coal surfaces of different roughnesses.The inset pictures were selected from the key points(points A to G)of the force curve.Initially,the water droplet approached the coal surface without any recorded force (point A).Subsequently,the droplet established contact with the surface (point B) and spread immediately with the recorded force (point C).After being continually pressed by 0.3 mm (point D),the droplet stretched (point E) owing to the retraction of the coal.The droplet is split(point G)when the critical diameter of the water bridge(point F)was reached.Because of the hydrophilicity of the surface(θYL<90°),all the droplets split.However,before the critical diameter of the water bridge appeared,the maximum adhesion force preventing the detachment of water droplets was recorded at point E,which was significant in the force curve.The recorded interaction forces at key points (points C,E,and F)between the water droplet and the coal surface with different roughness values are shown in Fig.7.

Fig.6.Force curves between water droplets and coal surfaces with different roughness.

Fig.7.Forces between water droplets and coal surfaces at different points.

As shown in Fig.6,after the water droplets were in contact with the coal surface and pressed,almost all of them remained on the surface.A comparison of the pictures at point E revealed that the contact line length increased with roughness;hence,it can be deduced that line pinning(L-constant line)would be more obvious on a rougher surface,resulting in a higher adhesion force between the water and surface [34].Comparing the pictures at point G,the ultimate water droplet angles were consistent with the results of the contact angle measurement,i.e.,the contact angle decreased with the surface roughness.As shown in Fig.7,after contact was established,the instantaneous spreading of the droplet (point C)generated a spreading force that increased with roughness because of the improved hydrophilicity.When retracting,the maximum adhesion force (point E) appeared on the curve,and the force varied with roughness with the same tendency.Moreover,when the critical diameter of the water bridge appeared,the pull-off force recorded at point F was based on the same rules as those of points C and E.According to the Wenzel model,a smoother surface can be obtained by polishing to improve the hydrophobicity of the coal,thereby enhancing the floatability of coal.The results of contact angle were in good agreement with adhesion force measurements verifying that the adhesion/detachment force measurement between water droplet and coal surface can be adopted to indicate the floatability of the coal [48].

3.2.2.Air bubble on coal surface

Floatability is reflected by the difficulty in adhesion between minerals and bubbles [49].The interaction between a bubble and coal surface can directly reflect the floatability of coal.Fig.8 shows the force curves of the bubble interacting with coal surfaces of different roughnesses.Unlike the water droplets,the bubbles did not exhibit instantaneous spreading or adhesion after being in contact with the surface,and no attachment was observed.It can be inferred that the spreading force of the bubble was insufficient to be distinguished from environmental disturbance;hence,these values were meaningless.The corresponding forces at points E and F are shown in Fig.9.

Fig.8.Force curves between air bubbles and coal surfaces with different roughness.

Fig.9.Forces between air bubbles and coal surfaces at different points.

Based on the results of the interaction between the water droplet and coal surface,it can be inferred that the hydrophobicity of the coal surface decreased with increasing roughness.As shown in Fig.8,when the surface became smoother,the three-phase contact line was longer at point E,and the bubble was stretched more significant before it detached at point F.Furthermore,a microbubble was separated from the bubble and attached to the smoothest coal surface(point G),indicating the high hydrophobicity of its surface after being polished.And the corresponding adhesion force was 97.14,42.76,and 17.86 μN when the surface roughness was 0.23,0.98,and 2.79 μm,respectively (Fig.9).Consistent with the results above,a lower adhesion force was recorded on the rougher surface owing to the filling of water in scratches.The stronger adhesion force between the water and coal surface compared with that between the bubble and surface resulted in a driving force that restricted the extension of the three-phase contact line between the bubble and coal surface.The wetting state of the coal can be described by the Wenzel model.By increasing the roughness of the surface with θYL<90°,the apparent contact angle decreased and the three-phase contact line decreased because of the appearance of a water pocket.A higher adhesion force indicates a better floatability of the coal,which is conducive to flotation.Moreover,it can be inferred that with the increase in force before the bubble separated from the surface,the possibility of the bubbles being detached from particles decreases,which facilitates the improvement in flotation efficiency [50].

3.2.3.Oil droplet on coal surface

In general,it is difficult to separate coal by relying on its natural hydrophobicity.The collector is the critical reagent in froth flotation;hence,a comprehensive understanding of the interaction between oil and coal is important.The interaction curves between the oil droplets (diesel oil) and coal surface are shown in Fig.10,and the interaction forces are shown in Fig.11.

As depicted in Fig.10,when the oil was in contact with the coal,part of it separated from the oil droplet and its volume decreased as the roughness increased.This indicates that increasing the roughness can result in less oil being adsorbed on the coal surface,causing a lower efficiency of its modification.Furthermore,Fig.10 shows that initially,the force curves were similar to the air bubble curves,albeit some differences in the shape.It is speculated that the oil droplet with the lower elasticity and higher viscosity may generate more moderated deformation,and hence,the curves would have no significant peak points but a gentle incline compared with the bubbles and water droplets during the pull-off processes.By analyzing the adhesion force columns shown in Fig.11,it was discovered that the highest adhesion force(169.48 μN) was obtained on the smoothest surface and the lower adhesion forces(145.84 and 121.02 μN) were recorded on the rougher surfaces,being consistent with the trend in Fig.10.Generally,a greater adhesion force implies the easier spreading of oil on the surface;consequently,the coal will be more hydrophobic and the flotation efficiency enhanced.

3.3.Effect of roughness on floatability of coal

The interaction force (F) appearing on the droplet/bubbleattached surface is governed by the capillary force(FC)and Laplace pressure force (FL) [51].

wherertpcis the radius of the three-phase contact line;γ the liquid–vapor interfacial tension;θ the contact angle;Athe droplet/bubble base projected contact area(A=πr2tpc);and ΔPthe Laplace pressure described by Young-Laplace equation,as calculated by Eq.(5).

whereDxandDyare the principal radii of the liquid–vapor interface.

It is clear from Eq.(1)thatrtpcis positively correlated withF.For an ideal surface,the three-phase contact line can be defined asltpc=2πrtpc,whereas for the true surface,ltpcmay be higher or lower than the calculated value.The maximum adhesion forces between the droplet/bubble and coal surface are shown in Fig.12.As the roughness increased,the adhesion force between the water and coal surface increased owing to the increase in the three contact lines and the appearance of line pinning [52].Contrary tendencies were observed in the oil droplets and air bubbles because of the driving force of water,confirming that decreasing the surface roughness can aid in improving the floatability of coal.By increasing the roughness of the hydrophilic coal surface(θ<90°),the water droplet can be trapped in the grooves,thereby reinforcing the contortion of the water contact line such thatltpc>2πrtpc;however,this is difficult to quantify.Compared with the water droplets,the bubbles detached from the coal surface more easily because of the shorter contact line.It can be inferred that during the attachment process,it was difficult for the bubbles to extrude water from the grooves such thatltpc<2πrtpc.Meanwhile,based on a preliminary comparison of adhesion forces between the water/oil droplets and bubbles,it is speculated that by modifying the reagent,a significant amount of water can be expelled easier from the grooves,thereby increasing the effective contact area and facilitating a strong adhesion.

Fig.10.Force curves between oil droplets and coal surfaces with different roughness.

Fig.11.Forces between oil droplets and coal surfaces at different points.

Fig.12.Maximum adhesion force between droplet/bubble and coal surface.

The floatability of coal varies with its rank.It has been reported that the roughness of coal decreases as its rank increases,and that the surface of low-rank coal is the roughest[53,54].Furthermore,it has been acknowledged that the abundance of oxygen-containing groups in low-rank coal results in poor floatability and based on this study,coal with a rougher surface will further deteriorate the flotation process [55].Hence,to reduce the adverse effect of roughness,a higher stirring intensity should be adopted to yield stronger turbulence,which can promote rubbing between the particles and water,thereby decreasing the surface roughness.

4.Conclusion

The contact angle measurement indicated that increasing the roughness of the coal could make its apparent contact angle lower,leading to poorer floatability.The direct measurements of adhesion forces for water droplets,air bubbles,and oil droplets being in contact with coal surface revealed the influence of roughness on the coal floatability by quantitative analysis.The results can be summarised as follows.

(1) The adhesion forces between water droplets and coal surface were 111.70,125.48,and 136.42 μN with increasing roughness.It could be inferred that forces increased due to the increased contact line and appearance of line pinning.

(2) Under the liquid environment,the highest adhesion force(97.14 μN) was recorded on the smoothest surface(0.23 μm).As the surface roughness increased to 0.98 μm,the adhesion force decreased to 42.76 μN,and the force was 17.86 μN when the coal surface roughness up to 2.79 μm.

(3) According to adhesion force measurements between oil droplet and coal surfaces,the adhesion forces recorded on the roughest surface was 121.02 μN.Being polished by 400-mesh and 4000-mesh sandpapers,the coal surface became more hydrophobic as the adhesion forces between oil droplets and coal surface increased to 145.84 and 169.48 μN,respectively.Decreasing the roughness could promote the spreading of oil and strengthen the adhesion between bubble and coal surface which is beneficial to improve flotation efficiency.

Acknowledgements

This work was supported by the Jiangsu Natural Science Fund-Youth Fund (BK20190639),National Nature Science Foundation of China (Nos.21978318,51904300,and 51922106),and National Key R& D Program of China (2020YFC1908803).