Effects of micro- and nano-bubbles on the flotation separation of unburned carbon from coal fly ash

ABSTRACT

Nanobubble technology has found extensive use in enhancing the flotation efficiency of various minerals. The utilization of nanobubbles in flotation encompasses bulk nanobubbles, surface nanobubbles, and microbubbles; however, a comprehensive comparison of their respective roles is lacking. Furthermore, there is a dearth of studies on employing nanobubble systems in the flotation of coal fly ash. The coal fly ash characters were initially studied to reveal its ultrafine size (d₅₀: 24.80 μm) and porous structure that includes a multitude of hydrophilic functional groups. These attributes contribute to the inadequate hydrophobicity of unburned carbon within the ash. Subsequently, the effects of different collector types, frother types, and air flow rates on the efficiency of carbon removal using conventional flotation methods were examined. Comparative analysis showed that conventional flotation using diesel oil outperformed that which employed kerosene as the collector. Additionally, higher carbon removal efficiency was achieved using MIBC, with a longer carbon chain length, as the frother, as opposed to sec-Octyl alcohol. Furthermore, a direct relationship between air flow rate and coal fly ash flotation performance was observed – higher air flow rates corresponded to improved flotation outcomes. Finally, a comparison between conventional flotation and nanobubble-assisted flotation (utilizing bulk nanobubbles, surface nanobubbles, and micro-nanobubbles) was conducted. Remarkably, micro-nanobubble flotation yielded the most promising results, with the lowest loss-on-ignition in flotation tailings (3.15%) and the highest removal efficiency of unburned carbon (85.01%). These findings satisfied the loss-on-ignition requirements of Grade I fly ash standards.

KEYWORDS:

• Surface nanobubbles
• bulk nanobubbles
• froth flotation
• unburned carbon
• fly ash
• micro-bubbles

Acknowledgements

The authors (Guangxi Ma and Xiangning Bu) gratefully acknowledge the financial support from the Natural Science Foundation of Shandong Province, China (Grant No. ZR2023QE103) and the National Natural Science Foundation of China (Grant No. 52204296).

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Author Statement

Guangxi Ma: Conceptualization, Data curation, Formal analysis, Resources, Writing – original draft, Writing – review & editing, Supervision; Hao Huang: Data curation, Writing – original draft; Shuo Liu: Data curation, Writing – original draft; Xiangning Bu: Conceptualization, Software, Supervision, Visualization, Writing – review & editing; Dongping Tao: Conceptualization, Writing – review & editing; Muidh Alheshibri: Conceptualization, Writing – review & editing.

Additional information

Funding

The work was supported by the National Natural Science Foundation of China [52204296]; financial support from the Natural Science Foundation of Shandong Province, China [ZR2023QE103].