XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2783
film, which would encourage surrounding liquid to flow
toward the film, thus slowing down film thinning or even
enlarging film thickness (i.e., film thickening). This phe-
nomenon was found for foam films formed from various
types and concentrations of surfactants and electrolytes.
The dynamic stabilization effect of acoustic sound on foam
films can be explained by the hydrodynamic pressure. More
details are given elsewhere (Ng et. al, 2020b).
In contrast, Figure 13b shows that a wetting film con-
fined between a bubble and a slightly hydrophobized solid
surface can be destabilized by acoustic sound (Yang, et al
2024). A destabilization of the wetting film should indicate
a higher tendency of bubble-particle attachment, which is
in favor of improving flotation efficiency.
It was, therefore, suggested that the observed improve-
ment of coal flotation or mineral flotation could be due
to the cooperative effect of improved froth stability and
enhanced bubble–particle interaction.
DISCUSSION
The results presented hitherto suggested that it would be
possible to use acoustic sound to improve flotation recov-
ery without sacrificing product grade or improve product
grade without losing flotation recovery. Also, using acoustic
sound to improve flotation has been demonstrated to sig-
nificantly improve coarse particle flotation, and it is applica-
ble to different types of flotation machines in operation. Its
application can potentially cut reagent dosage significantly
in coal flotation and the added energy cost seems modest,
with potential significant contribution to improvement of
metallurgical coal flotation yield and decrease of the prod-
uct ash content for reducing value chain greenhouse gas
emissions (Scope 3), especially greenhouse gas emissions in
ironmaking and steelmaking.
The loudspeaker can be placed in air above the flota-
tion cell or within the flotation cell (either in the froth
phase or in the pulp phase). An advantage of using under-
water speaker is the significantly lower noise level, which
contributes to improvement of plant personnel’s health and
safety. The background noise level in the lab was 69 dB, the
loudspeaker (in air) above the flotation cell 120–125 dB,
and the underwater speaker (in the pulp phase) was 77 dB.
When placing the loudspeaker inside the flotation cell,
the displacement of slurry by the loudspeaker in the flota-
tion system may cause the flotation recovery to change as
the rising bubbles would be forced to rise past the speaker.
Possible effects of the presence of loudspeaker on flotation
were examined, and there was no statistically significant dif-
ference in the flotation recovery caused by placing the loud-
speaker within a flotation cell, suggesting that the spatial
presence of the underwater loudspeaker in the flotation
cell had no influence on the flotation performance. Note
that no froth crowding effect caused by the presence of the
speaker on flotation performance was observed. However,
the presence of large loudspeaker(s) may narrow down the
passage for particle, bubbles or bubble-particle aggregates,
which needs to be further investigated.
A striking advantage of this approach is that the local
stabilization of foam in a process by sound would not have
any impact on the downstream processes, which is sel-
dom achievable by taking traditional approaches such as
adjusting frother type/dosage or aeration rate or both. This
approach can potentially solve the overfrothing issue faced
by a flotation operation while maximising flotation effi-
ciency. It is unclear as to whether there are possible inter-
plays of loudspeaker type and its installation position and
orientation, reagents, and particle property, which is sub-
ject of on-going research.
The observed opposite trends of the stabilities of wet-
ting film and foam film caused by the acoustic sound might
be linked to the difference in the boundary condition: the
foam film has two free air-water interfaces while the wetting
film has only one free interface. Further fundamental stud-
ies in this direction is expected to elucidate the underlying
reasons for the observed trends and contribute to flotation
modelling and simulation.
Further test work at larger scales is in progress to estab-
lish the scaling-up factor for the flotation technology with
using low-frequency acoustic sound.
CONCLUSIONS
Use of low-frequency sound (350 Hz–400 Hz, at rather
modest additional energy cost) in flotation was developed
and assessed at laboratory scale and pilot scale in batch mode
and semi-continuous mode. The tested minerals included
coking coal, quartz and a copper ore. This approach can
potentially be used to:
• help inhibit bubble coalescence in flotation
• promote bubble-particle attachment
• locally stabilize froth within the flotation cell and
improve flotation recovery /kinetics /product grade
• reduce required reagent consumption.
If approved at industrial scale, this approach can improve
flotation operation efficiency and smoothness and poten-
tially contribute to the sustainability of mineral processing
operations.
film, which would encourage surrounding liquid to flow
toward the film, thus slowing down film thinning or even
enlarging film thickness (i.e., film thickening). This phe-
nomenon was found for foam films formed from various
types and concentrations of surfactants and electrolytes.
The dynamic stabilization effect of acoustic sound on foam
films can be explained by the hydrodynamic pressure. More
details are given elsewhere (Ng et. al, 2020b).
In contrast, Figure 13b shows that a wetting film con-
fined between a bubble and a slightly hydrophobized solid
surface can be destabilized by acoustic sound (Yang, et al
2024). A destabilization of the wetting film should indicate
a higher tendency of bubble-particle attachment, which is
in favor of improving flotation efficiency.
It was, therefore, suggested that the observed improve-
ment of coal flotation or mineral flotation could be due
to the cooperative effect of improved froth stability and
enhanced bubble–particle interaction.
DISCUSSION
The results presented hitherto suggested that it would be
possible to use acoustic sound to improve flotation recov-
ery without sacrificing product grade or improve product
grade without losing flotation recovery. Also, using acoustic
sound to improve flotation has been demonstrated to sig-
nificantly improve coarse particle flotation, and it is applica-
ble to different types of flotation machines in operation. Its
application can potentially cut reagent dosage significantly
in coal flotation and the added energy cost seems modest,
with potential significant contribution to improvement of
metallurgical coal flotation yield and decrease of the prod-
uct ash content for reducing value chain greenhouse gas
emissions (Scope 3), especially greenhouse gas emissions in
ironmaking and steelmaking.
The loudspeaker can be placed in air above the flota-
tion cell or within the flotation cell (either in the froth
phase or in the pulp phase). An advantage of using under-
water speaker is the significantly lower noise level, which
contributes to improvement of plant personnel’s health and
safety. The background noise level in the lab was 69 dB, the
loudspeaker (in air) above the flotation cell 120–125 dB,
and the underwater speaker (in the pulp phase) was 77 dB.
When placing the loudspeaker inside the flotation cell,
the displacement of slurry by the loudspeaker in the flota-
tion system may cause the flotation recovery to change as
the rising bubbles would be forced to rise past the speaker.
Possible effects of the presence of loudspeaker on flotation
were examined, and there was no statistically significant dif-
ference in the flotation recovery caused by placing the loud-
speaker within a flotation cell, suggesting that the spatial
presence of the underwater loudspeaker in the flotation
cell had no influence on the flotation performance. Note
that no froth crowding effect caused by the presence of the
speaker on flotation performance was observed. However,
the presence of large loudspeaker(s) may narrow down the
passage for particle, bubbles or bubble-particle aggregates,
which needs to be further investigated.
A striking advantage of this approach is that the local
stabilization of foam in a process by sound would not have
any impact on the downstream processes, which is sel-
dom achievable by taking traditional approaches such as
adjusting frother type/dosage or aeration rate or both. This
approach can potentially solve the overfrothing issue faced
by a flotation operation while maximising flotation effi-
ciency. It is unclear as to whether there are possible inter-
plays of loudspeaker type and its installation position and
orientation, reagents, and particle property, which is sub-
ject of on-going research.
The observed opposite trends of the stabilities of wet-
ting film and foam film caused by the acoustic sound might
be linked to the difference in the boundary condition: the
foam film has two free air-water interfaces while the wetting
film has only one free interface. Further fundamental stud-
ies in this direction is expected to elucidate the underlying
reasons for the observed trends and contribute to flotation
modelling and simulation.
Further test work at larger scales is in progress to estab-
lish the scaling-up factor for the flotation technology with
using low-frequency acoustic sound.
CONCLUSIONS
Use of low-frequency sound (350 Hz–400 Hz, at rather
modest additional energy cost) in flotation was developed
and assessed at laboratory scale and pilot scale in batch mode
and semi-continuous mode. The tested minerals included
coking coal, quartz and a copper ore. This approach can
potentially be used to:
• help inhibit bubble coalescence in flotation
• promote bubble-particle attachment
• locally stabilize froth within the flotation cell and
improve flotation recovery /kinetics /product grade
• reduce required reagent consumption.
If approved at industrial scale, this approach can improve
flotation operation efficiency and smoothness and poten-
tially contribute to the sustainability of mineral processing
operations.