XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2225
declining ore grades, it becomes necessary to grind ores to
finer sizes for liberation, escalating energy costs. One way
to minimize the problem would be to increase the top size
of flotation feeds, which in turn creates another problem
associated with floating coarse particles. Coarse particles
readily drop off air bubbles due to poor liberation and large
inertia. One way to address this problem would be to use
stronger collectors to give larger contact angles and, hence,
provide stronger bubble-particle attachment forces.
In flotation, a particle collides with an air bubble, caus-
ing the latter to deform, creating a capillary pressure (pc) in
the wetting film formed between the two macroscopic sur-
faces. Since pc 0, the water in the film drains, and the film
thins. If the film thickness (h) reaches ~250 nm, film thin-
ning begins to be controlled by the disjoining pressure (Π),
which is created by the surface forces, i.e., electrical double-
layer (EDL), van der Waals (vdW), and hydrophobic (HP)
forces. In flotation, both the EDL and vdW forces are repul-
sive, while the HP force is attractive (Pan and Yoon, 2016
Huang and Yoon, 2019). The wetting film ruptures when
the attractive HP force becomes stronger than the sum of
the EDL and vdW forces and forms a contact angle (q). As
is well known, the larger the contact angle, the faster the
kinetics of flotation and hence give rise to a higher flotation
recovery. In this regard, the contact angle is probably the
most important parameter in flotation. In effect, contact
angle formation may be considered as incipient flotation.
Without forming a contact angle, flotation is not possible.
That the vdW force is repulsive for bubble-particle
interactions is a major disadvantage of flotation, in which
air bubbles are used to selectively collect hydrophobic par-
ticles. If oil drops rather than air bubbles are used for the
same purpose, the vdW force becomes attractive (Huang
and Yoon, 2019), making it much easier for oil drops to
form larger contact angles. Figure 1 shows that an oil drop
(n-dodecane) forms a contact angle (q) of 171° with a C16
thiol-coated gold surface, while an air bubble forms q =91°.
That oil drops form so much larger contact angles than air
bubbles explains why TLF is superior to flotation (Huang
and Yoon, 2019).
The possibility of collecting fine particles from the
aqueous phase using oil drops rather than air bubbles was
explored in the 1960s by Lai and Fuerstenau (1968). They
used iso-octane to recover 0.1 µm alumina (Al2O3) par-
ticles using alkyl sulfonate as a hydrophobizing agent for
the colloidal particles. In this process, also known as two-
liquid flotation (TLF), hydrophobic particles are collected
as oil-in-water (o/w) emulsion droplets, with the particles
acting as a solid emulsifier. Thus, the process practically
has no lower particle size limit for flotation. The process
was further developed to remove entrained particles and
achieve high-grade concentrate grades (US patent No.
9,518,241, 2016). In the present work, the possibility of
recovering copper from CSTs using the TLF process has
been explored. Laboratory tests conducted on CST samples
produced high-grade (30–34%Cu) concentrates with high
recoveries. Simulation results obtained on the basis of the
laboratory test results show significant financial advantages
of recovering copper directly from CST using the TLF pro-
cess rather than sending it back to the rougher bank as CLs.
WORKING PRINCIPLE
In the TLF process, as schematically shown in Figure 2,
small drops of recyclable oil, e.g., hexane, are added to an
ore slurry in Step I to selectively collect hydrophobic parti-
cles and form o/w emulsion drops, while hydrophilic parti-
cles are rejected as waste slurry in Step II. The o/w emulsion
droplets (or agglomerates) move to a specially designed
device known as Morganizer, in which additional oil is
added to form water-in-oil (w/o) emulsion drops by phase
inversion in Step III. The w/o emulsion is then destabilized
using vibrating screens so that the hydrophobic particles are
fully dispersed in the oil phase, while the small water drop-
lets liberated from agglomerates become larger in size by
coalescence, fall to the aqueous phase at the bottom along
with the hydrophilic particles dispersed in water drops, and
are rejected as waste slurry. The hydrophobic particles dis-
persed in the oil phase are then separated from the organic
phase by solid-liquid separation, e.g., filtration, while the
residual oil left on the surface of hydrophobic particles is
Figure 1. Contact angles formed on a C16-coated gold with a) air bubble and b) oil
(dodecane) drop
declining ore grades, it becomes necessary to grind ores to
finer sizes for liberation, escalating energy costs. One way
to minimize the problem would be to increase the top size
of flotation feeds, which in turn creates another problem
associated with floating coarse particles. Coarse particles
readily drop off air bubbles due to poor liberation and large
inertia. One way to address this problem would be to use
stronger collectors to give larger contact angles and, hence,
provide stronger bubble-particle attachment forces.
In flotation, a particle collides with an air bubble, caus-
ing the latter to deform, creating a capillary pressure (pc) in
the wetting film formed between the two macroscopic sur-
faces. Since pc 0, the water in the film drains, and the film
thins. If the film thickness (h) reaches ~250 nm, film thin-
ning begins to be controlled by the disjoining pressure (Π),
which is created by the surface forces, i.e., electrical double-
layer (EDL), van der Waals (vdW), and hydrophobic (HP)
forces. In flotation, both the EDL and vdW forces are repul-
sive, while the HP force is attractive (Pan and Yoon, 2016
Huang and Yoon, 2019). The wetting film ruptures when
the attractive HP force becomes stronger than the sum of
the EDL and vdW forces and forms a contact angle (q). As
is well known, the larger the contact angle, the faster the
kinetics of flotation and hence give rise to a higher flotation
recovery. In this regard, the contact angle is probably the
most important parameter in flotation. In effect, contact
angle formation may be considered as incipient flotation.
Without forming a contact angle, flotation is not possible.
That the vdW force is repulsive for bubble-particle
interactions is a major disadvantage of flotation, in which
air bubbles are used to selectively collect hydrophobic par-
ticles. If oil drops rather than air bubbles are used for the
same purpose, the vdW force becomes attractive (Huang
and Yoon, 2019), making it much easier for oil drops to
form larger contact angles. Figure 1 shows that an oil drop
(n-dodecane) forms a contact angle (q) of 171° with a C16
thiol-coated gold surface, while an air bubble forms q =91°.
That oil drops form so much larger contact angles than air
bubbles explains why TLF is superior to flotation (Huang
and Yoon, 2019).
The possibility of collecting fine particles from the
aqueous phase using oil drops rather than air bubbles was
explored in the 1960s by Lai and Fuerstenau (1968). They
used iso-octane to recover 0.1 µm alumina (Al2O3) par-
ticles using alkyl sulfonate as a hydrophobizing agent for
the colloidal particles. In this process, also known as two-
liquid flotation (TLF), hydrophobic particles are collected
as oil-in-water (o/w) emulsion droplets, with the particles
acting as a solid emulsifier. Thus, the process practically
has no lower particle size limit for flotation. The process
was further developed to remove entrained particles and
achieve high-grade concentrate grades (US patent No.
9,518,241, 2016). In the present work, the possibility of
recovering copper from CSTs using the TLF process has
been explored. Laboratory tests conducted on CST samples
produced high-grade (30–34%Cu) concentrates with high
recoveries. Simulation results obtained on the basis of the
laboratory test results show significant financial advantages
of recovering copper directly from CST using the TLF pro-
cess rather than sending it back to the rougher bank as CLs.
WORKING PRINCIPLE
In the TLF process, as schematically shown in Figure 2,
small drops of recyclable oil, e.g., hexane, are added to an
ore slurry in Step I to selectively collect hydrophobic parti-
cles and form o/w emulsion drops, while hydrophilic parti-
cles are rejected as waste slurry in Step II. The o/w emulsion
droplets (or agglomerates) move to a specially designed
device known as Morganizer, in which additional oil is
added to form water-in-oil (w/o) emulsion drops by phase
inversion in Step III. The w/o emulsion is then destabilized
using vibrating screens so that the hydrophobic particles are
fully dispersed in the oil phase, while the small water drop-
lets liberated from agglomerates become larger in size by
coalescence, fall to the aqueous phase at the bottom along
with the hydrophilic particles dispersed in water drops, and
are rejected as waste slurry. The hydrophobic particles dis-
persed in the oil phase are then separated from the organic
phase by solid-liquid separation, e.g., filtration, while the
residual oil left on the surface of hydrophobic particles is
Figure 1. Contact angles formed on a C16-coated gold with a) air bubble and b) oil
(dodecane) drop