Floatablity Measurements

Motivation

Bubble–particle attachment is the central subprocess in froth flotation that determines whether valuable mineral particles can be recovered. Although flotation performance is often linked to particle hydrophobicity, the actual attachment process is governed by a broader set of coupled parameters, including particle size, particle concentration, surface chemistry, bubble surface mobility, contact time, and the local hydrodynamic conditions inside the flotation cell. By studying particle attachment directly on individual bubbles under controlled but flotation-relevant flow conditions, we aim to connect microscale attachment mechanisms with macroscopic flotation performance.

Goals

• Quantify particle attachment kinetics on single bubbles under controlled hydrodynamic conditions.
• Investigate the effects of particle size, hydrophobicity, collector concentration, particle number density, and stirring conditions on attachment efficiency.
• Develop dynamic bubble surface coverage as a quantitative measure of particle floatability.
• Extend optical methods toward opaque and dense suspensions using X-ray radiography.
• Explore external fields, such as ultrasound, as a tool to enhance particle–bubble attachment.

Techniques

• Model stirred cell for floatability measurements.
• Optical imaging and shadowgraphy of dynamic bubble surface coverage.
• Particle image velocimetry to characterize local flow fields.
• Collection and post-analysis of attached particles to determine mass and size distribution.
• Microfocus X-ray radiography for non-invasive visualization in dense or optically inaccessible suspensions.
• Ultrasound-assisted attachment experiments using standing acoustic waves.

Results

We developed a model stirred-cell approach in which the surface of a single bubble is directly observed while particles attach under defined flow conditions. The time-dependent surface coverage of the bubble provides a fast and reproducible measure of attachment dynamics and can be related to particle floatability. This method allows the effects of particle size, concentration, hydrophobicity, collector dosage, and local hydrodynamics to be separated more clearly than in conventional flotation tests.

The results show that bubble coverage is not controlled by surface chemistry alone. The local flow structure around the bubble strongly affects how particles encounter and attach to the interface. Depending on the bubble position in the stirred cell, downward and rotational flow components can contribute differently to the attachment rate. This is important for flotation, where bubbles experience spatially varying flow fields rather than idealized uniform motion. For mixed particle systems, the attachment behavior can become selective. In bimodal systems, smaller particles were preferentially collected, whereas monomodal systems showed nearly unchanged size distributions before and after collection. Ultrafine particles can also alter the attachment of larger fine particles by changing their distribution, mobility, and packing density on the bubble surface.

Microfocus X-ray radiography extends this work to conditions where optical access is limited, such as dense slurries or suspensions containing many ultrafine particles. The method enables continuous visualization of particle attachment without interrupting the flow and provides a route to quantify bubble surface loading and aggregate morphology.

Ultrasound-assisted experiments further show that acoustic fields can enhance attachment when the operating window is properly chosen. Secondary acoustic radiation forces can pull particles toward the bubble and increase surface coverage, while excessive acoustic streaming or overly strong sonication may reduce attachment. These results provide practical guidance for tuning acoustic intensity, exposure time, and background hydrodynamics before moving toward pilot-scale flotation applications.

Publications

• Eftekhari, M., Schwarzenberger, K., Schlereth, P., & Eckert, K. Dynamics of particle attachment in a model stirred cell: A new technique to characterize and quantify particle floatability. Minerals Engineering, 2024.
• Eftekhari, M., Lappan, T., Heitkam, S., Schwarzenberger, K., & Eckert, K. X-ray radiographic investigation of particle–bubble attachment: Development of an integrated experimental–numerical methodology. 
• Keshmiri, A., Eftekhari, M., Heitkam, S., & Eckert, K. Ultrasound induced enhancement of bubble–particle attachment in flotation: A study on micron-sized model particles. Minerals Engineering, 2025.