Particle aggregates induced by nanobubble (NB) bridging has been recognized as one of the main contributors to improve the flotation performance of minerals in NBs-associated flotation. Despite the numerous investigations in particle-NB interaction, scarce research on revealing the properties of aggregates induced by NBs has been conducted, severally limiting the further understanding of the NB-assisted mineral flotation. In this study, NBs were first produced based on the principle of hydrodynamic cavitation. The aggregates of ultra-fine coal particles with different hydrophobicity induced by these tiny bubbles was investigated, with the size distribution, structure, and strength features of the aggregates systematically explained accordingly. In this study, NBs remained stable in the aqueous solution for more than one hour, and the size of these bubbles expanded from 240 to 650 nm within this period. The aggregates of coal particles was significantly enhanced, especially for the more hydrophobic ones, which could be well reflected by the right-shifted particle-size distribution curve, the newly formed chain-structure aggregates, and larger apparent viscosity and shear yield stress of the coal pulp. Moreover, the flotation performance (both flotation recovery and flotation kinetics) of ultra-fine coal particles was remarkedly improved in the presence of NBs, confirming that NBs did benefit the ultra-fine coal aggregates.
Ruxia Chen,Xianshu Dong,Zeyu Feng,Yuping Fan,Xiaomin Ma
Effective coal slurry water solid–liquid separation is indispensable for the recycling and sustainable development of coal resources. The interaction between bubble and coal particles plays a critical role in the process of dewatering for clean coal. In this study, we firstly conducted a comprehensive investigation of the impact of froth on the interactions between coal particles by rheological measurement and particle aggregation behavior. Furthermore, the macroscopic dewatering performance of coal slurry in the presence of froth and its microscopic cake structure were investigated using the filtration test and X-ray microtomography (CT). It was found that the interaction between coal particles in the presence of froth was enhanced as a result of the dynamic shear value, combined with the large floc size and compact structure, which led to a higher cake moisture and higher filtration velocity. The CT results indicated that the enhanced interaction of particles in the presence of froth also led to a dense microstructure of the filter cake. The porosity of the filter cake decreased to 2.05% when the aeration time increased from 0 s to 90 s, the throat radius in the filter cake was reduced to 1.32 μm, and the number of throat passages was reduced to one third. Multiple blind pores and low coordination numbers led to a poor connectivity of the pore network and high moisture content.
Understanding the collision between kerosene droplets and coal particles in the bubble trailing vortex region is crucial for enhancing slurry conditioning. We found the area of the bubble trailing vortex region increased as the bubble size increased via the Fluent 6.3.26 software. A kerosene droplet velocity range of 0.074–0.106 m/s and a coal particle velocity range of 0.062–0.104 m/s were obtained if a bubble had a diameter of 0.3 mm. We employed a high-speed motion acquisition system and observed the collision process, which was divided into two stages: Compression and Rebound . The critical motion distance increased as the kerosene droplet size increased, and sufficient kerosene droplet motion distance and enough kinetic energy normally resulted in a rebound phenomenon. The attenuation energy was almost positively linear to the initial energy, and the energy attenuation coefficient of the collision process was fitted to 0.626. Our results can provide valuable insight into mineral separation.
Even if coal particles are hydrophobic, interaction between these particles and air bubbles can be enhanced by nonpolar oil in froth flotation . The role of a nonpolar liquid dodecane on coal particle-bubble interaction in aqueous phase was investigated and compared to that of a solid analogue, paraffin wax via a combined application of water contact angle and particle-bubble stability measurements, and molecular dynamics simulations. It was found both dodecane and paraffin wax can increase hydrophobicity of coal surface and attachment stability of coal particle-bubble aggregates when they covered the coal particles. Despite similar chemical structure and hydrophobicity, dodecane was more effective than paraffin wax. Molecular simulations confirmed that, comparing to the unmodified coal particle, both dodecane and paraffin wax models could increase the attachment strength of coal particle. However, different from immobile solid paraffin wax, we demonstrated that the liquid like properties of the dodecane layers present on the coal particle surfaces play an essential role in the particle-bubble interaction. When the coal particle covered by dodecane arrived at gas−water interface, the dodecane molecules will migrate and rearrange to form a new film at this interface. In addition to hydrophobic attraction of the gas bubble, the dodecane film will provide additional adhesion towards the coal particle due to oleophilicity of the coal particle, enhancing the stability of bubble-particle aggregate.
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