Search

Flotation is the common method for fine low rank coal upgrading while it is always inefficient due to the rough surface morphology and rich oxygen-containing functional groups. Hydrothermal pretreatment is widely used in dewatering for low rank coal. The physicochemical properties of coal surface considerably vary following the hydrothermal pretreatment. In this study, hydrothermal pretreatments at different temperatures for different times were applied to enhance lignite flotation. The hydrothermal pretreatment at 200 °C can significantly improve the flotation efficiency, while the temperature raised to 300 °C, the flotation recovery did not increase or even decrease. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Zeta potential, Brunauer–Emmett–Teller, and particle size distribution analysis were conducted to explain the underlying mechanism. The contents of pore and crack on coal surface considerably decreased and more fresh and smooth surfaces exposed following hydrothermal pretreatment, according to SEM analysis. The FTIR spectroscopy and XPS showed that the contents of oxygen-containing functional groups were decreased, while the hydrophobicity of lignite was increased following pretreatment. The decrease of Zeta potential also indicated the decrease of oxygen-containing functional groups and increase of hydrophobicity on coal surface. However, excessive temperature of hydrothermal pretreatment could increase the pore diameter and fine particle fraction, leading to a higher collector consumption and lower bubble–particle collision efficiency. Consequently, the flotation recovery was not improved or even decreased at the hydrothermal pretreatment temperature of 300 °C.

The interaction force between particle and air bubble/water droplet is the key to understand the underlying mechanism for mineral flotation. The most commonly used approach is colloidal probe atomic force microscopy (AFM), which usually operates in the nano-Newton range. This makes the quantification of the adhesion (capillary) force during retraction challenging, because the magnitude of the adhesion force is much higher than that of surface forces. In this short communication, a new instrument, the micro-mechanical testing machine (MMTM), was used to measure the interaction force between a micrometer glass bead and an air bubble/water droplet with both micro-Newton range and nano-Newton resolution. A single-axis capacitive microforce sensor based on interdigitated comb capacitors was equipped in the MMTM. Similar to the colloidal probe AFM, a hydrophilic glass bead with a diameter of 193 μm was adhered to the end of the cantilever. The MMTM allows the simultaneous recording of the data of force, displacement, and time at a fixed velocity. The results showed that a strong jump-into contact and adhesion force were observed between the particle and water droplet. In contrast, a weaker jump-into and smaller adhesion force were observed between the bubble and particle. Based on the theoretical equation of the adhesion force, the equilibrium contact angle of the glass bead used was back calculated to be approximately 39.58°, which is reasonable for its force behavior. The MMTM provides a flexible and alternative way to investigate the surface and interfacial forces in flotation systems.

Surface roughness plays a critical role in flotation, as it has a marked impact on bubble-particle interaction, i.e., attachment and detachment. In the present study, the effect of surface roughness on the detachment between bubble and glass beads with different contact angles was investigated using a home-built system for measuring bubble-particle detachment. By applying a simple harmonic oscillation to a bubble-particle aggregate, the critical detachment force could be calculated from the critical amplitude based on Newton's second law. Smooth and rough glass beads were hydrophobized simultaneously using octadecyl trichlorosilane vapor to ensure that they have the same local contact angle in parallel experiments. The results show that the detachment force of a smooth particle was higher than that of a rough particle when the local contact angle with the particle surface was below 90° (hydrophilic). However, the detachment force of a rough particle was higher than that of a smooth particle when the local contact angle was above 90° (hydrophobic). Surface roughness rendered the hydrophilic bubble-particle aggregate less stable while hydrophobic bubble-particle aggregate more stable. The decrease in apparent contact angle and contact line length decreased the detachment force of the hydrophilic particles, while an increase in these parameters increased the detachment force of the hydrophobic particles.

As a sub-process of flotation, the bubble-particle detachment plays a critical role in final flotation recovery. In this short communication, the effect of vibration mode, i.e., sine wave, triangle wave and square wave, on the detachment of low-rank coal particle from oscillating bubble was investigated using a home-made bubble-particle detachment measurement system. The critical detachment amplitude was used to evaluate the stability of bubble-particle aggregate. Three coal size fractions, i.e., 0.25–0.5, 0.5–1, and 1–1.5 mm, were used as the experimental samples, respectively. The results show that the critical amplitude decreased as particle size increased under different vibration modes due to the increased inertia. The critical amplitude at three different size fractions always decreased in the following order: sine wave, triangle wave and square wave, indicating the bubble-particle aggregate was the most stable under sine wave vibration. Square wave vibration has the most dramatic movement path changes, causing a highest instantaneous impulse and detachment force.

Surface roughness has a great impact on coal floatability, and usually the effect is found to be negative. In this paper, hypobaric treatment was introduced to improve the floatability of coal with varying surface roughness. The floatability was characterized by induction time, and the micro-bubble nucleation theory was proposed to explain the enhancement mechanism of hypobaric treatment. The results show that the induction time was increased with increasing surface roughness. However, hypobaric treatment was proven to be an effective method in mitigating the adverse effects of surface roughness on floatability. Under natural conditions, neither Wenzel nor Cassie model could be used to describe the wetting regime of coal surface. Instead, a mixed wetting state, where water partially wets the asperities on coal surface and partially sits on entrapped air pockets, was more appropriate. It was difficult to drain the entrapped water at the upper part of the grooves; hence, bubble-coal attachment was prevented, leading to the increased induction time at high surface roughness. After hypobaric treatment, the diffused gas molecules in water preferred to nucleate at the sites of air pockets, leading to the formation of micro-bubbles. These micro-bubbles would act as the bridge between macroscopic bubbles and the coal surface, significantly decreasing induction time and improving the coal contact angle and floatability.

The presence of clay minerals presents a great challenge to fine coal and mineral flotation due to the well-known phenomena of mechanical coating and water entrainment. In this paper, we study the effect of two different types of clay, kaolinite and montmorillonite, on the floatability of coal in de-ionized water (without flotation reagents) based on induction time and bubble-particle attachment kinetics. The bubble-particle attachment angle (BPAA) as a function of time was used as the kinetics raw data. We demonstrated how the presence of montmorillonite was detrimental to the floatability of coal in de-ionized water. This was not the case with kaolinite. The induction time and BPAA kinetics of coal particles only changed slightly after conditioning together with kaolinite. In contrast, the induction time increased considerably from 8.73 ms to 83.25 ms when montmorillonite was added. In addition, both the maximum BPAA and BPAA kinetics constant decreased, and hydrophilic montmorillonite coating occurred during the conditioning stage. These hydrophilic sites on the coal surface prevent the thinning-rupture of the water film between bubble and coal particles and thus decrease the floatability and recovery.

In this work, the magnesium aluminum spinel (MgAl2O4) was prepared by Mg and Al as precursors through a method of sol-gel subsequent with high temperature calcination. The wide range of synthetic conditions, including organic alcohols, ammonia content, dispersant, alkoxide concentration, hydrolysis time, hydrolysis temperature and calcination temperature were screened over as-prepared samples. This work provides a necessary experimental basis for the synthesis of MgAl2O4 with uniform particle size of spherical structure, which has a potential to be used in many industrial and military applications.

It is well recognized that an improved flotation recovery can be achieved by introducing nanobubbles to common flotation practice due to the increased capture efficiency between bubbles and particles. However, the specific role of nanobubbles in bubble-particle interactions (collision, attachment, and detachment) is not well understood. In the present study, we explore the role of surface nanobubbles in bubble-particle detachment. Surface nanobubbles were introduced via ethanol-water exchange and their presence was confirmed using laser scanning confocal microscopy (LSCM). The effect of surface nanobubbles on bubble-particle detachment behavior was then investigated using an oscillating bubble apparatus. Bubble-particle aggregate stability was evaluated using critical detachment amplitude. Further, bubble-particle detachment forces in the absence and presence of nanobubbles were measured directly using a micro-nano mechanical testing system. Using LSCM, numerous surface nanobubbles were observed on a glass surface after ethanol-water exchange, regardless of wettability. The number and lateral dimensions of generated nanobubbles on the hydrophilic surface were significantly smaller than that on the hydrophobic surface. Surface nanobubbles increased the stability of bubble-particle aggregates. Macroscopic air bubbles coalesce with the nanobubbles on the particle surface, increasing the pinning effect of the three-phase contact line and advancing contact angle. As a result, the capillary force between bubbles and particles increased in the presence of surface nanobubbles.

The floatability of fine low-rank coal particles can be greatly influenced by their morphological characteristics, such as shape and surface roughness. In this study, the attachment efficiency and detachment amplitude of fine low-rank coal particles produced by various comminution methods onto/from the bubble surface were investigated using homemade bubble&ndash, particle wrap angle and bubble&ndash, particle attachment/detachment testing systems. Results showed that the length&ndash, diameter ratio of rod-milled products was smaller than that of crushed products. The wrap angle of particles obtained by the crushed method was larger than that obtained by the rod-milled method, i.e., particles with greater length&ndash, diameter ratio showed higher attachment efficiency onto the bubble surface. Meanwhile, particles with greater length&ndash, diameter ratio exhibited a larger detachment amplitude, which suggests that it is more difficult to be detached from the bubble surface. However, rod-milled products showed lower attachment onto the bubble surface. The flotation test confirmed that the floatability ratio of crushed products was higher than that of rod-milled products, consistent with evidence from experimental analyses. This study provides a fundamental understanding of particle shapes for low-rank coal flotation by a novel research method combining the attachment efficiency and detachment amplitude of bubble&ndash, particle combinations.

Bubble-particle detachment is the key sub-process for determining the upper size limit of flotation. In this study, vertical adhesion force between particle and different positions on a bubble surface were measured using a micro-mechanical testing machine (MMTM). Three micrometer glass beads with varying hydrophobicities (hydrophilic, medium hydrophobic, and highly hydrophobic) were adhered to the end of the force sensor, and an air bubble with a diameter of 950 μm was fixed at the bottom of the hydrophobic quartz cell. Different contact positions were selected by gradually increasing the offset distance from the vertical axis of the bubble. The results showed that the back-calculated advancing contact angles of the hydrophilic, medium hydrophobic, and highly hydrophobic particles were 45, 86, and 124°, respectively. The vertical adhesion force always decreased with increasing offset distance, regardless of particle hydrophobicity. The decrease in the vertical component of the normal capillary force at the contact point might be the main reason for the decreased vertical adhesion force. Additionally, a linear region in the retraction force curves was always observed, illustrating that the bubble behaved as a Hooke’s spring under a vertical force. The spring constants at different positions on the bubble surface were found to decrease with increasing offset distance, which were consistent with the variations in vertical adhesion force observed.