XXXI International Mineral Processing Congress 2024 Proceedings Page 2647 (2671 of 3836)

XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2781
meantime, use of the sound improved the combustible
recovery from 82.6% to 84.0%. Note that no independent
repeats were done because the study was limited by the
amount of coal samples available. Further work needs to be
done to examine whether the improvement is statistically
significant.
STABILITY OF FROTHS, FOAMS, FOAM
FILMS AND WETTING FILMS
Effect of Sound on Froth and Foam Stability
Figure 11 shows the stabilities of froths and foams in a lab-
scale flotation column (Ng et al., 2022). The variations in
froth stability and foam stability were strong correlated
with each other, implying that the mechanisms of the foam
and froth stabilizations could be similar. The preferred
sound frequency for achieving froth/foam stability would
be around 350 Hz, and at a given sound frequency, an
increase in the sound amplitude would increase the stability
of foam/froth, similar to the variation of flotation efficiency
shown in Figure 3.
Table 1. Effect of sound (at 400 Hz and 105 dB) on the
efficiency of the pilot-scale flotation in semi-continuous mode
Yield
(Mass Recovery)
Combustible
Recovery
Blank 59.8% 82.6%
Acoustic Sound 61.1% 84.0%
Source: Ng et al., 2022.
Figure 11. Effect of sound frequency and amplitude on froth stability represented by water
recovery (R
w )in quartz flotation) and foam stability represented by the maximum foam height
(h
eq ).The experimental setups are similar to what is shown in Figure 2b

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XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2781
meantime, use of the sound improved the combustible
recovery from 82.6% to 84.0%. Note that no independent
repeats were done because the study was limited by the
amount of coal samples available. Further work needs to be
done to examine whether the improvement is statistically
significant.
STABILITY OF FROTHS, FOAMS, FOAM
FILMS AND WETTING FILMS
Effect of Sound on Froth and Foam Stability
Figure 11 shows the stabilities of froths and foams in a lab-
scale flotation column (Ng et al., 2022). The variations in
froth stability and foam stability were strong correlated
with each other, implying that the mechanisms of the foam
and froth stabilizations could be similar. The preferred
sound frequency for achieving froth/foam stability would
be around 350 Hz, and at a given sound frequency, an
increase in the sound amplitude would increase the stability
of foam/froth, similar to the variation of flotation efficiency
shown in Figure 3.
Table 1. Effect of sound (at 400 Hz and 105 dB) on the
efficiency of the pilot-scale flotation in semi-continuous mode
Yield
(Mass Recovery)
Combustible
Recovery
Blank 59.8% 82.6%
Acoustic Sound 61.1% 84.0%
Source: Ng et al., 2022.
Figure 11. Effect of sound frequency and amplitude on froth stability represented by water
recovery (R
w )in quartz flotation) and foam stability represented by the maximum foam height
(h
eq ).The experimental setups are similar to what is shown in Figure 2b

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2780 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
were conducted owing to the difficulty of securing a large
amount of flotation feed sample. The froth depth was kept
at 14 cm and at steady state, samples of flotation feed, con-
centrate and tailing streams were collected within a pre-set
period of time to determine the ash content of the particles
in each stream, which in turn was used to determine the
flotation efficiency based on mass balancing.
Table 1 summarizes the flotation test results. The flota-
tion yield for the blank (without sound) was 59.8%, and
with the acoustic sound, it was improved to 61.1%. In the
Source: Yang et al., 2023b..
Figure 9. Improvement of pilot-scale coal flotation using acoustic sound
Figure 10. Schematic of the experimental setup used to conduct the flotation tests at pilot scale
in semi-continuous mode. The tailing stream was sent back to the feed sump. Loudspeakers were
placed within the flotation cell, approximately 5 cm below the flotation cell lip (but above the
pulp-froth interface)

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2782 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Figure 12 shows the stabilities of foams in a pilot-
scale flotation column. The experimental setup shown in
Figure 7 was used to identify the preferred location of the
loudspeaker for improving foam stability. The frother used
was polypropylene glycol (PPG) with an average molecu-
lar weight of 425 g/mol. The PPG solutions were prepared
using local tap water. The loudspeaker (Monarcor AR-50)
was used with the frequency set at 400 Hz. The foam stabil-
ity was indicated by the flowrate of the overflowing foam,
which should be somehow proportional to the air recovery,
an indicator for foam/froth stability (Hadler and Cilliers,
2009). Use of the sound systematically improved foam
stability, at any given frother concentration tested and any
location of the loudspeaker installed. It was observed that
the location of the loudspeaker would be preferably 5 cm
below the foam surface if the speaker was positioned less
than 4cm below the foam surface, it would tend to reduce
the foam stability if the speaker was located 15 cm (or
more) below the foam surface, the improvement of foam
stability would become smaller.
Effect of Sound on the Stability of Foam Films
(Representing Bubble-Bubble Interaction) and Wetting
Film (Representing Bubble-Particle Interaction)
Figure 13a shows that with sound at a certain frequency
and amplitude, the stability of foam films could be signifi-
cantly improved, with evolution of the foam films being
associated with vigorous liquid flows within the films. The
presence of vigorous liquid flows indicates a relatively high
kinetic energy and thus a relatively low pressure within the
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
5cm 10cm 15cm
Speaker's distance to foam top
a) 10 ppm PPG
Blank Sound
3.00
3.20
3.40
3.60
3.80
4.00
5cm 10cm 15cm
Speaker's distance to foam top
b) 50 ppm PPG
Blank Sound
Figure 12. Effect of sound (at 400 Hz) on foam stability (represented by the overflowing froth flowrate) with the loudspeaker
immersed in the foam at different positions. The tests were conducted at two frother concentrations using the setup shown in
Figure 7
Source: Ng et al., 2020b, Yang, 2023.
Figure 13. Characteristic images of liquid film evolution: a) foam film, b) wetting film
Overflowflowrate
(liter/min)
Overflow
flowrate
(liter/min)

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