2
main component of the mixing mechanism in forced-air
mechanical flotation machines is the rotor/stator assembly.
The rotor, which is considered to be the heart of a mechani-
cal flotation cell, as it provides the energy needed to per-
form a majority of its expected functions (Gorain, 2007), is
the central component that provides agitation and mixing.
It rotates to create turbulence, dispersing air and enhancing
contact between the particles and bubbles. On the other
hand, the stator, which is located around the impeller, helps
regulate the flow of slurry and air, promoting uniform mix-
ing and stability of the froth, as well as enhancing the shear-
ing environment around the rotor leading to fine bubble
creation. All these characteristics are related with achieving
the expected functions of a flotation cell.
Within a mechanical flotation machine, three hydro-
dynamic zones can be recognized (Gorain et al., 2000), as
shown in Figure 1, each playing a distinct role for effec-
tive flotation. The turbulent zone is where the rotor gener-
ates turbulence, mixing the pulp and gas effectively. In this
zone the rotor-stator mechanism keeps solids in suspen-
sion, disperses gas into small bubbles and promotes bub-
ble-particle interaction for valuable materials collection.
Above the turbulent zone, the quiescent zone provides a
calmer area where the bubble-particle aggregates move
upward in a region with relatively low turbulence, helping
to decrease non-valuable minerals becoming mechanically
entrained between bubbles. The froth zone, located above
the quiescent zone, acts as an final cleaning step, by reduc-
ing water, and subsequent entrained gangue particles, in
the concentrate. This zone is crucial for concentrating the
floated materials. This zone requires a fixed slurry height,
maintained by a level sensor and discharge valve, to ensure
a constant removal of the enriched froth, over the concen-
trate lip and out of the flotation cell, which results in con-
sistent concentrate quality.
The flotation mixing mechanism is intrinsically linked
to the hydrodynamic zonification within a mechanical flo-
tation machine, as it directly influences the behavior and
efficiency of each zone. Understanding these hydrodynamic
zones enables operators to optimize flotation conditions,
enhancing mineral recovery and improving the overall effi-
ciency of the process.
Harris (1976) emphasized that the main distinction
among flotation equipment manufacturers lies in the rotor
design. In the 1980s, there were many mechanical flotation
cell designs available from many different manufacturers
globally (Gorain, 2007). Today there are only a few major
ones, but design differences still remain.
Enhancing the design of flotation equipment can sig-
nificantly boost metallurgical performance, resulting in
higher grade and/or higher recoveries. This enhancement
can be achieved by optimized rotor-stator designs. In addi-
tion, considering that, on average, 68% of the total life-
cycle cost of large flotation cells is due to the lifetime energy
consumption cost (Rinne and Peltola, 2008), reduced
operating costs can be achieved through an optimal mix-
ing mechanism design. Therefore, it is incumbent upon
flotation cell manufacturers to continuously innovate and
improve their products.
METSO FLOTATION MIXING
MECHANISM DEVELOPMENT JOURNEY
Recently, Metso commemorated its 50-year legacy in
mechanical flotation. The journey from Outokumpu’s pio-
neering efforts –and then further developed by Outotec– to
Metso’s continuous pursuit of innovation has shaped the
course of mechanical flotation and undoubtably propelled
the industry forward (Rinne, 2023).
With respect to the flotation mixing mechanism, there
has been an extensive and continuous innovation effort by
Metso over the years, leading to widely utilized and indus-
try-recognized mechanism designs. FloatForce+ is the new-
est iteration of Metso’s rotor design and comes to extend
the successful legacy of the FloatForce mechanism. A sum-
mary of this journey is presented in Table 1.
Figure 1. Hydrodynamic zones in a mechanical flotation cell
(adapted from Gorain et al., 2000)
Table 1. Metso’s flotation mixing mechanism development
timeline
Launch year Mechanism design
1973 Outokumpu Multi-Mix
1990 Outokumpu Free-Flow
2005 FloatForce®
2024 FloatForce®+
main component of the mixing mechanism in forced-air
mechanical flotation machines is the rotor/stator assembly.
The rotor, which is considered to be the heart of a mechani-
cal flotation cell, as it provides the energy needed to per-
form a majority of its expected functions (Gorain, 2007), is
the central component that provides agitation and mixing.
It rotates to create turbulence, dispersing air and enhancing
contact between the particles and bubbles. On the other
hand, the stator, which is located around the impeller, helps
regulate the flow of slurry and air, promoting uniform mix-
ing and stability of the froth, as well as enhancing the shear-
ing environment around the rotor leading to fine bubble
creation. All these characteristics are related with achieving
the expected functions of a flotation cell.
Within a mechanical flotation machine, three hydro-
dynamic zones can be recognized (Gorain et al., 2000), as
shown in Figure 1, each playing a distinct role for effec-
tive flotation. The turbulent zone is where the rotor gener-
ates turbulence, mixing the pulp and gas effectively. In this
zone the rotor-stator mechanism keeps solids in suspen-
sion, disperses gas into small bubbles and promotes bub-
ble-particle interaction for valuable materials collection.
Above the turbulent zone, the quiescent zone provides a
calmer area where the bubble-particle aggregates move
upward in a region with relatively low turbulence, helping
to decrease non-valuable minerals becoming mechanically
entrained between bubbles. The froth zone, located above
the quiescent zone, acts as an final cleaning step, by reduc-
ing water, and subsequent entrained gangue particles, in
the concentrate. This zone is crucial for concentrating the
floated materials. This zone requires a fixed slurry height,
maintained by a level sensor and discharge valve, to ensure
a constant removal of the enriched froth, over the concen-
trate lip and out of the flotation cell, which results in con-
sistent concentrate quality.
The flotation mixing mechanism is intrinsically linked
to the hydrodynamic zonification within a mechanical flo-
tation machine, as it directly influences the behavior and
efficiency of each zone. Understanding these hydrodynamic
zones enables operators to optimize flotation conditions,
enhancing mineral recovery and improving the overall effi-
ciency of the process.
Harris (1976) emphasized that the main distinction
among flotation equipment manufacturers lies in the rotor
design. In the 1980s, there were many mechanical flotation
cell designs available from many different manufacturers
globally (Gorain, 2007). Today there are only a few major
ones, but design differences still remain.
Enhancing the design of flotation equipment can sig-
nificantly boost metallurgical performance, resulting in
higher grade and/or higher recoveries. This enhancement
can be achieved by optimized rotor-stator designs. In addi-
tion, considering that, on average, 68% of the total life-
cycle cost of large flotation cells is due to the lifetime energy
consumption cost (Rinne and Peltola, 2008), reduced
operating costs can be achieved through an optimal mix-
ing mechanism design. Therefore, it is incumbent upon
flotation cell manufacturers to continuously innovate and
improve their products.
METSO FLOTATION MIXING
MECHANISM DEVELOPMENT JOURNEY
Recently, Metso commemorated its 50-year legacy in
mechanical flotation. The journey from Outokumpu’s pio-
neering efforts –and then further developed by Outotec– to
Metso’s continuous pursuit of innovation has shaped the
course of mechanical flotation and undoubtably propelled
the industry forward (Rinne, 2023).
With respect to the flotation mixing mechanism, there
has been an extensive and continuous innovation effort by
Metso over the years, leading to widely utilized and indus-
try-recognized mechanism designs. FloatForce+ is the new-
est iteration of Metso’s rotor design and comes to extend
the successful legacy of the FloatForce mechanism. A sum-
mary of this journey is presented in Table 1.
Figure 1. Hydrodynamic zones in a mechanical flotation cell
(adapted from Gorain et al., 2000)
Table 1. Metso’s flotation mixing mechanism development
timeline
Launch year Mechanism design
1973 Outokumpu Multi-Mix
1990 Outokumpu Free-Flow
2005 FloatForce®
2024 FloatForce®+