Water droplets and air bubbles at magnesite nano-rough surfaces: Analysis of induction time, adhesion and detachment using a dynamic microbalance

doi:10.1016/j.mineng.2020.106449

Minerals Engineering

Volume 155, 15 August 2020, 106449

https://doi.org/10.1016/j.mineng.2020.106449 Get rights and content

Highlights

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Magnesite surfaces having RMS = 2 to 240 nm roughness were produced via polishing.
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Attachment, adhesion and detachment of water droplets and air bubbles were recorded.
•
Induction time increased and adhesion force decreased for bubbles on rough magnesite.
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Nano-roughness enhances the adhesive strength of water with magnesite surface.

Abstract

Surface roughness affects the interactions of solids with droplets and bubbles. In this study, natural magnesite lumps were polished by a series of sandpapers and diamond to produce four magnesite specimens having 2 to 240 nm root-mean-square roughness. The dynamic measurements of attachment, spreading, adhesion, and separation with a high-sensitivity microelectronic mechanical balance revealed the effect of surface nano-scaled roughness on the induction time and forces of spreading, adhesion and separation for both water droplets and air bubbles. It was found that the increasing nano-scaled roughness enhances the spreading of water on hydrophilic magnesite and strengthens the water-magnesite adhesive contact. Nano-roughness also causes delays in attachment of air bubbles to magnesite surface, inhibits displacement of water by adhering air bubbles, and reduces the adhesive strength of air bubbles to the magnesite surface, factors that might slow down the flotation separation.

Graphical abstract

Introduction

Flotation is a complex physicochemical process involving at least three phases (solid, liquid and gas), and was invented to selectively separate targeted particles from other particles suspended in the pulp (Crabtree and Vincent, 1962). Selectivity of flotation separation is driven by differences in particle natural or induced hydrophobicity, combined with a control over the particle–particle and particle-bubble colloidal interactions (Chau et al., 2009, Drelich and Marmur, 2018). Although the process may appear relatively simple, there are up to 100 variables that can impact the flotation process (Shean and Cilliers, 2011).

The effect of surface roughness on flotation of mineral particles has been in focus among several research laboratories in the last two decades, but the published reports often lead to contradicting conclusions, as already reviewed in the previous paper (Guven et al., 2015). For example, Ulusoy and Yekeler (2005) reported that the increased surface roughness leads to a reduced water contact angle and floatability of quartz, talc, barite, and calcite particles. In another study, Hicyilmaz et al. (2005) reported enhanced floatability of barite particles with smoother surfaces in the presence of A-845 (Cytec) succinamate surfactant; autogenous milling was used to lower both barite particle roughness and acuteness. Ahmed (2010), on the other hand, found that particles with rough surfaces, containing a larger number of micro-structural defects, stabilize the froth, and improve the flotation kinetics. Guven et al. (2015) studied the influence of particle roughness on flotation of methylated glass beads and demonstrated that particle surface roughness benefits the flotation recovery. Later, Hassas et al. (2016) confirmed this finding for glass beads but using hexadecyl trimethyl ammonium bromide as a collector. Li et al. (2019) also demonstrated the facilitating effect of micro-sized surface roughness on the floatability of malachite particles. The effect of surface roughness on the bubble-particle induction time was also analyzed. Chen et al., 2018, Xia, 2017) measured the attachment time for air bubbles in contact with natural coal particles. The authors reported that as surface roughness increases, the attachment time increases and at the same time, the bubble-coal adhesion contact area decreases. Xing et al. (2019) additionally reported that hypobaric treatment can mitigate the adverse effects of surface roughness on attachment time.

Although the effects of particle surface roughness on flotation and bubble-particle attachment are reported in the literature quite well, understanding the factors that drive these effects appears incomplete yet. In AFM colloidal force measurements, Karakas and Hassas (2016) demonstrated higher repulsions between smooth particles as compared to rough particles. Feng and Aldrich (2000) reported that the rough surfaces with a high concentration of microstructural defects provided more active centers for accelerated dissolution of particles, benefiting the adsorption of reagents onto the particle surfaces, and leading to enhance flotation performance. Theoretical modeling revealed that nano-scaled asperities can reduce energy barriers in interactions between rough particles and gas bubbles (Drelich, 2018, Drelich and Bowen, 2015). Micro-scaled roughness, on the other hand, does not have the same effect, and it can stabilize a water film, particularly for hydrophilic substrates with the water contact angle is less than 65–70° (Drelich et al., 2011).

Previously, we demonstrated the positive effect of particle nano-scaled roughness on flotation recovery of magnesite (Zhu et al., 2020). We also speculated through theoretical modeling that nano-scale asperities lower energy barriers during particle-bubble interactions. In this study, we examine directly the effect of nano-scaled magnesite surface roughness on the attachment, spreading, adhesion and separation of both the water droplets and air bubbles to understand whether there are other factors than energy barriers that could be accounted to flotation performance of rough particles. The measurements were carried out using the high-sensitivity microelectronic mechanical balance equipped with a camera and data acquisition software, which can instantly record the adhesion force and image a shape of the droplet/bubble, from which both contact angles and dimensions are extracted. The results reveal that with increasing surface nano-scaled roughness, the adhesion of water increases, delaying attachment of air bubble and decreasing bubble adhesion.

Section snippets

Mineral samples

High quality magnesite lumps were selected from an ore mined in Dandong, Liaoning Province, China. The x-ray fluorescence analysis confirmed a high purity of specimens with more than 97 wt.% MgCO₃. The magnesite lumps were cut into approx. 10 × 10 × 5 mm specimens using a metallographic saw. The grinding and polishing of surfaces were carried out by abrasive sandpaper of 120, 400, and 1200 mesh and 1 μm diamond powder. The polished magnesite samples were washed with ethanol and deionized water

Surface roughness

Fig. 3 shows the representative 3-dimensional (3D) AFM images of 10.0 × 10.0 μm² magnesite surfaces after grinding and polishing. The 3D AFM images confirmed a reduction in surface roughness of mineral surface after each step of polishing using the sandpaper with increasing mesh number and then using a fine diamond powder. The average values for surface roughness parameters (R_q and R_a) and surface area ratio (R_SA), together with their standard deviation values, are listed in Table 1.

The R_q and R

Conclusions

The direct measurements of attachment, spreading, adhesion, and separation for water droplets and air bubbles in contact with magnesite surfaces support the followings conclusions:

1)
Magnesite mineral is hydrophilic, showing a preferential affinity of this mineral to water rather than to air, with the most stable (water) contact angle of 25–32 degree as determined experimentally and calculated based on the Wenzel equation.
2)
Nano-scaled roughness of magnesite surface enhances adhesive strength with

CRediT authorship contribution statement

Zhanglei Zhu: Methodology, Validation, Investigation, Writing - original draft. Donghui Wang: Resources. Bin Yang: Investigation. Wanzhong Yin: Supervision, Conceptualization. Jaroslaw W. Drelich: Supervision, Conceptualization, Writing - review & editing, Visualization, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The financial supports for this work from the National Natural Science Foundation of China under the grant No. 51874072, the Fundamental Research Funds for the Central Universities under the grant No. N180106006 are gratefully acknowledged. Zhanglei Zhu also appreciates the China Scholarship Council (No. 201906080099) for financial support of his visiting study to Michigan Technological University, USA.

Financial support for purchasing the atomic force microscope at Michigan Tech through

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Citation Excerpt :
Lump magnesite polished by 120 mesh sandpaper had the roughest surface, followed by lump magnesite polished by 400 mesh and 1200 mesh sandpapers, while the lump magnesite polished by 1 μm diamond was smoothest (Ni et al., 2018). From our prior study (Zhu, Wang, Yang, Yin, & Drelich, 2020), it was revealed that Rq roughness value decreased from 240 ± 26 nm for magnesite polished with 120 mesh sandpaper to 69 ± 12 nm for magnesite polished with 400 mesh sandpaper to 42 ± 8 nm for magnesite polished with 1200 mesh sandpaper to 2.3 ± 0.2 nm for magnesite polished with 1 μm diamond. As evidenced by Table 3, after interacting with 100 mg/L NaOL, the advancing contact angles of lump magnesite surfaces polished by 120, 400, 1200 mesh sandpapers and 1 μm diamond were 124.3°, 121.6°, 118.3° and 114.3°, respectively, illustrating that rougher magnesite particles exhibited better hydrophobicity, which agreed well with the previous study (Li et al., 2019; Wang & Zhang, 2020).
Surface roughness has a significant influence on mineral flotation. The assisting effect of surface roughness on minerals flotation is extensively investigated from its physical properties (e.g., the existing form of asperity and its size), however, the associated effect on mineral flotation based on the differences in surface chemical property caused by surface roughness has been rarely touched. With such a question in mind, in this study, we investigated the flotation recoveries of two batches of magnesite particles with varying degree of surface roughness produced by two different mills, and associated the flotation performances to their surface chemical properties (amount of adsorption sites for the collector) via a series of detections, including Scanning Electron Microscope-Energy Dispersive Spectrometry (SEM-EDS) observations, X-ray photoelectron spectroscopy (XPS) analysis, adsorption capacity tests, and contact angle measurements. Finally, we concluded that rougher magnesite particles could provide more active sites (Mg²⁺) for a larger capacity of sodium oleate (NaOL), thereby improving the hydrophobicity and floatability.

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Water droplets and air bubbles at magnesite nano-rough surfaces: Analysis of induction time, adhesion and detachment using a dynamic microbalance

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Mineral samples

Surface roughness

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgement

Int. J. Miner. Process.

Adv Colloid Interface Sci

Fuel

Adv. Colloid Interface Sci.

Int. J. Miner. Process.

Miner. Eng.

Int. J. Miner. Process.

Miner. Eng.

Acta Biomater.

Colloids Surf., A

Int. J. Miner. Process.

Surf. Innovations

Colloids Surf., A

Chem. Eng. Process. Process Intensif.

Int. J. Miner. Process.