Seher Ata

Various bubble generation methods have been developed to produce microbubbles, but current techniques are inadequate for meeting industrial demands for controlling the size of microbubbles accurately. This study aimed to investigate acoustic bubble generation as a potential solution to the demand. Initially, a capillary tube was exposed to a continuous standing wave to control the bubble generation, which resulted in a relatively large bubble size and number. However, the extent of bubble coalescence was high due to the attractive secondary acoustic radiation force (ARF) between vibrating bubbles. Alternatively, a pulsed wave could reduce attractive ARF, thereby simultaneously reducing bubble coalescence and controlling the bubble generation frequency and size. However, this approach compromised the quantity of bubbles generated. The research contributes to the mass production of microbubbles and the strategic control of bubble size to optimize process efficiency.

Filtration is crucial for separating solids and liquids in various industries. Understanding slurry properties and filter cake structures is essential for optimising filtration performance. Conventional methods focus on interpreting filtration data to improve the understanding of filtration mechanisms. However, examining fragile filter cakes is challenging, and current techniques often alter their structure. Conventional methods only provide an average representation of cake porosity, neglecting variations in porosity and pore distribution across the cake thickness. This study introduces the Digital cake analysis, a non-destructive technique for evaluating filter cake structure. Filtration experiments using a custom-built unit were conducted on coal slurries to obtain filter cake samples. X-ray-microcomputed tomography (µCT) imaging was utilized for cake analysis. Image enhancement techniques were employed to improve the quality of the µCT images. The enhanced images were segmented into three phases (resolved pore, subresolution pore, and solid phases) for quantitative analysis. This segmentation technique allocated partial pore volume to voxels in the subresolution phase based on their intermediate grey-scale intensity, enabling more accurate porosity calculations. Unlike conventional methods, this approach computed porosity values for resolved (100% void) and subresolution (partially void) pores. This image segmentation technique facilitated accurate computations of porosity, pore size distribution, and pore properties, significantly advancing the understanding of cake structures. Digital cake analysis produced porosity measurements similar to the experimental results.

This paper analyses the basic obstacles preventing the fine particles from floating and suggests solutions for the wetting zone between the bubble and the particle during their collision. It has been shown in our recent paper that the basic problem of fine particle flotation is not the low frequency of collisions with the bubbles, but it consists of the efficiency of these collisions. Moreover, there exists a thermodynamic lower size limit for flotation of fine hydrophobized particles in the sub-micron range, and it is weakly dependent on the size of the bubbles. It was shown that fast flotation with high recovery of fine particles can be achieved by means of: (i) electrostatic attraction between particles and bubbles; (ii) a significant increase in the level of their hydrophobicity; (iii) existence of fine bubbles in the flotation cell. It was shown as well that the drainage of the wetting film between bubbles and particles is unimportant, but the deformation of the bubble by the particle during their clash plays a major role in its rupturing. Electrostatic attraction between bubbles and fine silica particles was achieved with hexylamine. It causes a moderate increase of their hydrophobicity from contact angle = 39.5° ± 2.5° to contact angle = 51.7° ± 7.5° and gave almost 90% recovery within 2 min. Unfortunately, the selectivity of this collector is unsatisfactory if the fine silica particles are mixed with fine magnesite particles. It was shown that even being hydrophilic, the recovery of fine particles can jump to almost 50% if strong electrostatic attraction with the bubbles exists. It was demonstrated as well with the collector hexamethyldisilazane causes significant increase of the hydrophobicity of the fine silica particles (contact angle ≈ 90°) results in skin flotation with 100% recovery when alone and 97% recovery when being mixed with fine magnesite particles (51/49). A new collector significantly increasing the hydrophobicity of magnesite fine particles was tested (disodium dodecyl phosphate) resulting in 89% recovery of fine magnesite particles alone and about 98% recovery in a mixture with fine silica particles.

The behavior of moving bubbles has mostly been studied in an axisymmetric flow field. To extend the knowledge to practical conditions, we investigate the interfacial and hydrodynamic properties of bubbles under asymmetric shear forces. Experiments are performed with a buoyant bubble at the tip of a capillary placed in a defined shear flow in the presence of surfactants, nanoparticles, and glass beads. The response of the interface to the surrounding asymmetric flow is measured under successive reduction of the surface area. Profile analysis tensiometry is utilized to investigate the dynamic surface tension and the surface rheology of the surfactant- and nanoparticle-laden interfaces. Microscopic particle image and tracking velocimetry are used to study the bulk flow and the interfacial mobility of the buoyant bubble. According to our results, the rotational component of the shear flow provokes an interfacial flow, which redistributes the adsorbed surfactants and particles at the interface. In the presence of NPSCs, a contiguous network of particles forms at the interface through densification of surface structures. We show that this interconnected nanoparticle network eventually stops the interfacial flow and decreases the mobility of the glass beads at the interface. The immobilization of the interface is characterized by a dimensionless number, defined as the ratio of the interfacial elasticity to bulk shear forces. This number provides an estimate of the interfacial forces required to impose interfacial immobility at a defined flow field. Our findings can serve as a basis to formulate boundary conditions for refined modeling and to predict the hydrodynamics of bubbles and droplets.

Mining operations often send samples for testing to commercial laboratories. Unless a customised test is requested, they expect laboratories to use standard procedures, which are reproducible. A thermal coal and a metallurgical coal were sent to eight laboratories, which were requested to perform a basic flotation test (AS 4156.2.1-2004) and a sequential flotation procedure test, i.e., standard tree test (AS 4156.2.2-1998). This study compared the reports produced by the various laboratories and compared them with the requirements laid out by the Australian standards. It was found that many elements were missing in most cases, probably due to the fact that some of the requirements of the standard, such as size analysis, are offered as other services. The basic tests generally agreed with one another whilst the sequential tests presented more variations. A quantitative analysis of the variation in the yield–ash curves produced by the sequential procedure was conducted using dynamic time warping (DTW). This approach can be used to numerically compare yield–ash curves and perform statistical comparisons.

The motion of particles in a monolayer induced by the coalescing of a bare bubble with a planar air–water interface was investigated in a modified Langmuir trough. Experiments were performed to understand the effect of particle hydrophobicity, subphase pH, packing density, the presence of a weak surfactant, and particle size distribution on the behavior of particle movement in the monolayer during the coalescence process. Video tracking software was used to track the particles and extract data based on the video footage. Visual inspection indicated that the coalescence of the bubble with the monolayer was a chaotic process which led the interface to oscillate to an extent that the particles underwent complete rearrangement. A simple analysis was carried out on the main forces involved in particle motion and rearrangement at the oscillating air–water interface. The motion characteristic of particles was evaluated by speed and mean-square displacement (MSD). The results showed that the butanol-treated particles had higher speed and MSD than the particles with a stronger affinity to the air–water interface. Similar results were also found at high subphase pH and low packing factor.

Coal preparation plants are increasingly turning to alternative sources of water to reduce their consumption of fresh water. These alternative sources can vary significantly in ionic content. This review discusses the effect of dissolved inorganic electrolytes on coal flotation. It is argued that increasing process monitoring will enable a better understanding of the effects of salts on the flotation process. A pragmatic approach in monitoring is essential to differentiate significant factors from insignificant ones controlling the performance of coal flotation units in salt water. The effects of inorganic electrolytes on the air–water interface and the solid–liquid interface are reviewed to assist in the interpretation of industrial results.