XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1765
Basin involved a complex process of crushing, grinding,
and chemical processing to extract the gold from the ore.
The tailings from this processing operation are a mixture
of crushed rock, water, and the chemical reagents used for
gold extraction. These tailings were stored in large tailings
dams, which now cover thousands of hectares of land in
the Witwatersrand region. The tailings contain a signifi-
cant amount of gold that could not be extracted using the
technology available at the time, and they have become a
valuable resource for gold recovery using modern process-
ing methods.
Ultrasound technology has been proposed as a poten-
tial solution for improving the recovery of gold from these
tailings. When mechanical waves frequency exceeds the
upper audible limit of human hearing (20 kHz) they are
regarded as ultrasound waves. The propagation of ultra-
sound waves in a liquid medium causes bubble generation
as the result of periodic cycles of rarefaction (regions of low
pressure) and compression (regions of higher pressure),
(Mason, et al., 2002). During the low pressure cycle, the
bubbles formed undergoes growth phase until they reach
a critical size where they can no longer absorb energy and
then implode on the next successive high pressure cycle.
The bubble implosion comes with the generation of micro
jets and shock waves and this process is known as acoustic
cavitation. Micro jet is a small (typically less than 1 mm
in diameter), high speed stream of fluid produced when a
bubble collapse that can impact nearby surfaces with a high
energy density. A shock wave is a very large localized pres-
sure perturbation which travels at a speed slightly greater
than the speed of sound (343 m/s). These shock waves
generated amount to localized pressures of about 1000
atm. Furthermore, the rapid adiabatic compression of gases
and vapours within the bubbles lead to the production of
localized high pressures and temperatures of thousands of
degrees Celsius, (Luque-García &Luque de Castro 2003).
Microjets produced from bubble implosion bombard the
solid surfaces at higher speed of around 100 m/s leading
to the exfoliation of the solid surfaces. Thus, micro jets can
help with the exposure of new surface zones of the solid
subsequently enhancing contact of mineral grains with
leaching agents during sono-leaching.
Beside the production of shock waves and micro jets,
literature also suggests that free radicals are generated dur-
ing the cavitation process especially when ultrasound is
irradiated with low frequency (from a few dozens to several
hundred kHz), (Luque-García &Luque de Castro 2003).
During the process of cavitation, the extremely high tem-
perature and high pressure produced by the implosion of
cavitation bubbles leads to the thermal dissociation of water
molecules, which in turn results in generation of radicals,
·OH and ·H. The ·OH radical is a very strong oxidizing agent
(Eo =2.80 V) with ability to oxidize minerals such as pyrite
which is the main inclusion in Witwatersrand refractory
gold tailings. Acoustic cavitation generates extreme condi-
tions of turbulence which are able to effectively disrupt the
boundary layer around the gold particles and increase the
contact area between the solid and liquid phases. Again, the
acoustic streaming induced by ultrasound plays a crucial
role in enhancing mass transfer. Acoustic streaming refers
to the steady flow patterns created by the propagation of
ultrasound waves. The resulting convective flow increases
the transport of reactants and products to and from the
gold surface, facilitating the diffusion of leaching agents and
accelerating the dissolution of gold particles. Furthermore,
ultrasound-assisted leaching can modify the physicochemi-
cal properties of the leaching system. The application of
ultrasound waves can alter solution pH, temperature, and
redox potential, thereby influencing the kinetics and ther-
modynamics of gold dissolution. These changes can opti-
mize the leaching conditions and promote the extraction of
gold from refractory ores or complex matrices.
In gold leaching, passivation can also occur when the
surface of gold particles becomes coated with a layer of
reaction products or other substances, which can inhibit
further leaching of the gold. Ultrasound can avoid passiv-
ation during gold leaching by enhancing mass transfer, pro-
ducing free radicals and promoting cavitation. If the surface
of the gold particle is passivated, the rate of gold leaching
will decrease significantly. Ultrasound can help to overcome
passivation by promoting the production of free radicals
through cavitation, which can break down the passivating
substances, enabling the leaching agent to access the gold
surface, (Collasiol et al., 2004). The energy provided by the
ultrasonic waves can prevent the agglomeration of particles
and enhance the transport of the leaching agent to the par-
ticles’ surfaces, leading to an increase in the gold leaching
rate. This makes ultrasound technology suitable for appli-
cations in diffusion controlled leaching processes where
leaching kinetics is slow because of the formation of prod-
uct layers on the surface of the particles to be leached, or in
ores where gold occurs as inclusion pyrite (Barrera-Godinez
et al., 1992). In this research, ultrasound technology will be
applied on the pretreatment and cyanide leaching for gold
extraction from Witwatersrand tailings.
EXPERIMENTAL
Sampling and Analysis
A sample of gold leach tailings from a gold processing plant
in South Africa was used for all the experimental work. A
Basin involved a complex process of crushing, grinding,
and chemical processing to extract the gold from the ore.
The tailings from this processing operation are a mixture
of crushed rock, water, and the chemical reagents used for
gold extraction. These tailings were stored in large tailings
dams, which now cover thousands of hectares of land in
the Witwatersrand region. The tailings contain a signifi-
cant amount of gold that could not be extracted using the
technology available at the time, and they have become a
valuable resource for gold recovery using modern process-
ing methods.
Ultrasound technology has been proposed as a poten-
tial solution for improving the recovery of gold from these
tailings. When mechanical waves frequency exceeds the
upper audible limit of human hearing (20 kHz) they are
regarded as ultrasound waves. The propagation of ultra-
sound waves in a liquid medium causes bubble generation
as the result of periodic cycles of rarefaction (regions of low
pressure) and compression (regions of higher pressure),
(Mason, et al., 2002). During the low pressure cycle, the
bubbles formed undergoes growth phase until they reach
a critical size where they can no longer absorb energy and
then implode on the next successive high pressure cycle.
The bubble implosion comes with the generation of micro
jets and shock waves and this process is known as acoustic
cavitation. Micro jet is a small (typically less than 1 mm
in diameter), high speed stream of fluid produced when a
bubble collapse that can impact nearby surfaces with a high
energy density. A shock wave is a very large localized pres-
sure perturbation which travels at a speed slightly greater
than the speed of sound (343 m/s). These shock waves
generated amount to localized pressures of about 1000
atm. Furthermore, the rapid adiabatic compression of gases
and vapours within the bubbles lead to the production of
localized high pressures and temperatures of thousands of
degrees Celsius, (Luque-García &Luque de Castro 2003).
Microjets produced from bubble implosion bombard the
solid surfaces at higher speed of around 100 m/s leading
to the exfoliation of the solid surfaces. Thus, micro jets can
help with the exposure of new surface zones of the solid
subsequently enhancing contact of mineral grains with
leaching agents during sono-leaching.
Beside the production of shock waves and micro jets,
literature also suggests that free radicals are generated dur-
ing the cavitation process especially when ultrasound is
irradiated with low frequency (from a few dozens to several
hundred kHz), (Luque-García &Luque de Castro 2003).
During the process of cavitation, the extremely high tem-
perature and high pressure produced by the implosion of
cavitation bubbles leads to the thermal dissociation of water
molecules, which in turn results in generation of radicals,
·OH and ·H. The ·OH radical is a very strong oxidizing agent
(Eo =2.80 V) with ability to oxidize minerals such as pyrite
which is the main inclusion in Witwatersrand refractory
gold tailings. Acoustic cavitation generates extreme condi-
tions of turbulence which are able to effectively disrupt the
boundary layer around the gold particles and increase the
contact area between the solid and liquid phases. Again, the
acoustic streaming induced by ultrasound plays a crucial
role in enhancing mass transfer. Acoustic streaming refers
to the steady flow patterns created by the propagation of
ultrasound waves. The resulting convective flow increases
the transport of reactants and products to and from the
gold surface, facilitating the diffusion of leaching agents and
accelerating the dissolution of gold particles. Furthermore,
ultrasound-assisted leaching can modify the physicochemi-
cal properties of the leaching system. The application of
ultrasound waves can alter solution pH, temperature, and
redox potential, thereby influencing the kinetics and ther-
modynamics of gold dissolution. These changes can opti-
mize the leaching conditions and promote the extraction of
gold from refractory ores or complex matrices.
In gold leaching, passivation can also occur when the
surface of gold particles becomes coated with a layer of
reaction products or other substances, which can inhibit
further leaching of the gold. Ultrasound can avoid passiv-
ation during gold leaching by enhancing mass transfer, pro-
ducing free radicals and promoting cavitation. If the surface
of the gold particle is passivated, the rate of gold leaching
will decrease significantly. Ultrasound can help to overcome
passivation by promoting the production of free radicals
through cavitation, which can break down the passivating
substances, enabling the leaching agent to access the gold
surface, (Collasiol et al., 2004). The energy provided by the
ultrasonic waves can prevent the agglomeration of particles
and enhance the transport of the leaching agent to the par-
ticles’ surfaces, leading to an increase in the gold leaching
rate. This makes ultrasound technology suitable for appli-
cations in diffusion controlled leaching processes where
leaching kinetics is slow because of the formation of prod-
uct layers on the surface of the particles to be leached, or in
ores where gold occurs as inclusion pyrite (Barrera-Godinez
et al., 1992). In this research, ultrasound technology will be
applied on the pretreatment and cyanide leaching for gold
extraction from Witwatersrand tailings.
EXPERIMENTAL
Sampling and Analysis
A sample of gold leach tailings from a gold processing plant
in South Africa was used for all the experimental work. A