Yujin Sun

Surface roughness directly affects the interaction between mineral surfaces and water, as well as the adhesion of particles to bubbles during mineral flotation processes. Currently, there is a significant amount of research on the relationship between mineral surface roughness and wettability, yet the conclusions drawn are not consistent. To investigate the impact of roughness on the adhesion and detachment behavior between droplets and coal surfaces, this study prepared a series of coal samples with varying roughness using sandpaper of different grit sizes. A highly sensitive adhesion force measuring instrument was employed to study the continuous attachment and detachment processes between droplets and coal surfaces of different roughness levels. Contact angle results indicated that as the roughness of the coal sample surface increased, the contact angle gradually decreased, suggesting an increase in the hydrophilicity of the coal surface. This study proposed a concavo-convex roughness model for predicting the contact angle on coal surfaces, which was validated for feasibility and accuracy through experimental data. The adhesion force between droplets and coal surfaces increased with roughness. As the roughness increased from 0.30 μm to 2.39 μm, the spreading force of the droplet increased from 159.00 μN to 209.60 μN, the maximum adhesion force increased from 406.76 μN to 441.08 µN, and the detachment force increased from 95.37 μN to 102.39 μN. A smaller contact angle between the droplet and the coal surface corresponded to a larger contact diameter and greater interaction force. The forces measured by the adhesion force measurement device showed good consistency with theoretical calculations. This study provides theoretical support for understanding the interaction processes between droplets and rough solid surfaces.

With the severe depletion of coarse flake graphite (a critical raw material) resources, developing and utilizing fine and ultrafine graphite resources have recently attracted attention. Froth flotation is a widely used technique for the initial enrichment of graphite; however, the flotation selectivity decreases significantly along with particle size reduction. Ultrasound pretreatment would be a promising method to improve the flotation of fine particles. As an innovative approach to understand better the flotation response of different flake graphite sizes, this study conducted a comparative analysis based on flotation concentrate yield and ash as well as ash removal rate between the flake graphite with various particle sizes after ultrasound pretreatment. Particle size, X-ray powder diffraction, and scanning electron microscopy and energy dispersive X-ray spectroscopy analyses were used to investigate the effect of ultrasound treatment on mineralogical properties of the flake graphite with varied particle sizes. Process outcomes indicated that the flotation performance of fine flake graphite (mean chord length: 62.63 μm) was significantly enhanced after ultrasound pretreatment. However, flotation of the ultrafine flake graphite (mean chord length: 24.97 μm) after ultrasound treatment was limited due to the difficulty of generating sufficient fragmentation and dissociation by microjets and shock waves formed by the cavitation effect. Compared with conventional flotation, the concentrate yield of ultrasound flotation increased from 88.95 to 94.98%, ash content decreased from 5.72 to 4.87%, and ash removal rate enhanced from 36.94 to 42.61%. Particle size and mineral property analyses confirmed that further crushing and dissociation of the larger flake graphite after ultrasound pretreatment would be the main factors contributing to improved flotation performance. Additionally, the formation of air flocs in the coarse flake graphite during the ultrasound pretreatment process facilitated the flotation recovery of the crushed graphite particles.

The recovery of difficult-to-float coal using traditional nonpolar hydrocarbon oil collectors can be challenging, particularly for low-rank or oxidized coal. Thus, there is a need for more efficient flotation agents. Nanoparticle flotation collector technology has become increasingly popular in the field of mineral processing, and the presence of various ions in the slurry can significantly affect the interaction between collectors and mineral surfaces. In this study, cationic polystyrene (PS) nanoparticles were prepared using an emulsion polymerization method, and the effects of Na⁺ ion concentration on the in situ adsorption and desorption processes, adsorption layer configuration, and adsorption kinetics of PS particles on amorphous carbon (coal model) and SiO₂ sensors (quartz mineral model) were analyzed using the quartz crystal microbalance with dissipation (QCM-D) technique. Our results showed that the hydrophobic PS nanoparticles irreversibly adsorbed onto both amorphous carbon and SiO₂ sensors under different environmental conditions, and their adsorption capacity decreased gradually with increasing Na⁺ ion concentration. Increasing Na⁺ ion concentration from 0 M to 1.0 M resulted in a 24.4% and 30.9% decrease in equilibrium adsorption capacities of PS nanoparticles onto amorphous carbon and SiO₂ surfaces, respectively. The adsorption rate of PS nanoparticles onto the SiO₂ surface was much greater than that on the amorphous carbon surface. The adsorption rate constant of PS nanoparticles onto SiO₂ surfaces was 0.782 at 0.1 M Na⁺ ion concentration, while its adsorption rate constant onto amorphous carbon surfaces was only 0.060. Moreover, the adsorption process was found to be more in line with the quasi-primary kinetic model. These findings suggest that PS nanoparticles may serve as promising flotation collectors for the recovery of difficult-to-float coal, and highlight the importance of considering the effect of dissolved ions on the adsorption properties of flotation collectors.

The flotation method is widely used for the preliminary beneficiation of aphanitic (microcrystalline) graphite. However, there is limited literature regarding the effects of flotation reagents on the flotation kinetics of aphanitic graphite. In this study, six commonly used flotation kinetic models were used to fit the flotation experimental data of aphanitic graphite. The classical first-order model was found to be most suitable for describing flotation kinetics of aphanitic graphite. The modified flotation rate constant (K_m) was then applied to evaluate the effects of collector, frother, and inhibitor on aphanitic graphite flotation kinetics. Compared to diesel oil and terpineol oil, kerosene and 2-octanol produced a greater K_m. The highest K_m was obtained at an inhibitor dosage of 15 mg/L.

Separation performance of mechanical flotation cell and cyclonic microbubble flotation column (FCMC) was compared in terms of the beneficiation of high-ash coal fines that has the characteristics of the content of η of 47.37%, the clean coal yield of 39.19% and the clean coal ash content of 10.78% at the feed pulp concentration of 40 g/L through twice explorative flotation experiments and the orthogonal experiment. It was found that the ash content of the clean coal had a strong dependence on the feed concentration. In contrast, the yield of the concentrate of the column flotation was increased to 43.26% with the lower ash content of 9.47% and the higher η of 54.51% even at higher feed concentration of 60 g/L. The remarkable separation efficient of the FCMC flotation column for the difficult-to-float coal slime that has the characteristics of high content of fine particles with high ash was attributed to the presence of the centrifugal force field, more microbubbles generated by hydrodynamic cavitation, the form of countercurrent mineralization, and thicker froth layer.