XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2949
Froth stability depends primarily on gas dispersion condi-
tions, but chemical conditions and particle properties such
as froth mobility and transport rate can be increased by
manipulating the froth zone.
Crowders and launders play an important role in the
froth flow and the behavior of the phases in the tank. A
launder is an open-topped channel into which the froth
is collected after overflowing a weir. The froth from the
launders on individual cells flows to a common launder or
pipe, and moves on to further processing. A crowder is a
structure fitted inside the tank to direct and concentrate the
froth flow to the launders.
Launders must be designed to carry the expected
amount of froth. The “lip” is the portion of the launder
over which the froth flows. The launder must have lip
length and volume sufficient to accommodate the antici-
pated froth flow, and the pumps and piping that transport
the froth to the next stage in the process must also be sized
correctly.
The froth carrying rate (FCR) is defined as the dry mass
of concentrate removed from the flotation cell, per square
meter of froth area, per hour (t/m2-hr). The FCR varies in
each stage of the flotation circuit: 0.8 to 1.5 in roughers,
0.3 to 0.8 in scavengers, and 1.0 to 2.0 in cleaners. If the
FCR is too low, the froth will be weak and will collapse eas-
ily. Transport of solids to the lip of the launder will be poor,
and the likelihood of particle drop-back will increase. If it
is too high, the froth may collapse from the weight of the
solids it is carrying. Again, solids transport to the launder
lip may be poor, but decreasing residence time in the froth
may limit drainage time, resulting in gangue entrainment.
Flotation residence time is naturally related to volume,
and gas flow rate through froth removal is related to surface
area. Because of this, as cells become larger, concentrate
flow rate does not increase proportionally to cell volume.
These constraints have led to the use of froth crowders on
virtually all cylindrical machines and to several alternative
launder designs for those machines, as shown in Figure 5.*
Coleman (2009) states that the selection of type, num-
ber, and size of flotation cells required to meet specific
processing needs for an operation depend on the required
flotation residence time and on the amount of concentrate
that can be recovered for a given overflowing lip length
and froth surface area. However, the total lip length and
the surface area at the top of the froth can be modified
by changing the configuration of crowders and launders.
Customizing this configuration is in fact considered a cost-
effective solution for modifying operating parameters such
as froth carry rate and froth lip loading without increasing
the number of flotation tanks.
In large, cylindrical machines, froth recovery is more
difficult for three reasons. First, the distance to a froth laun-
der on the perimeter of the machine is larger. Second, in
large cylindrical cells, the ratio of the surface area at the top
of the cell, through which the froth flows, may not increase
proportionally with increased volume, unless the aspect
ratio of the cylinder is kept constant. Third, the circumfer-
ence of the cell, over which the froth must flow for recovery,
increases as the square root of the volume increase.
MECANICALLY AGITATED CELLS
In the 1970s, as ore grades decreased, mining companies
increased throughput to decrease unit costs. At the same
*A detailed discussion of froth launders can be found in Nelson
and Lelinski 2019.
Figure 5. Left: Metso radial launder with center and peripheral launders (Metso 2024a) Right: FLSmidth radial crowders and
launders (International Mining 2024)
Froth stability depends primarily on gas dispersion condi-
tions, but chemical conditions and particle properties such
as froth mobility and transport rate can be increased by
manipulating the froth zone.
Crowders and launders play an important role in the
froth flow and the behavior of the phases in the tank. A
launder is an open-topped channel into which the froth
is collected after overflowing a weir. The froth from the
launders on individual cells flows to a common launder or
pipe, and moves on to further processing. A crowder is a
structure fitted inside the tank to direct and concentrate the
froth flow to the launders.
Launders must be designed to carry the expected
amount of froth. The “lip” is the portion of the launder
over which the froth flows. The launder must have lip
length and volume sufficient to accommodate the antici-
pated froth flow, and the pumps and piping that transport
the froth to the next stage in the process must also be sized
correctly.
The froth carrying rate (FCR) is defined as the dry mass
of concentrate removed from the flotation cell, per square
meter of froth area, per hour (t/m2-hr). The FCR varies in
each stage of the flotation circuit: 0.8 to 1.5 in roughers,
0.3 to 0.8 in scavengers, and 1.0 to 2.0 in cleaners. If the
FCR is too low, the froth will be weak and will collapse eas-
ily. Transport of solids to the lip of the launder will be poor,
and the likelihood of particle drop-back will increase. If it
is too high, the froth may collapse from the weight of the
solids it is carrying. Again, solids transport to the launder
lip may be poor, but decreasing residence time in the froth
may limit drainage time, resulting in gangue entrainment.
Flotation residence time is naturally related to volume,
and gas flow rate through froth removal is related to surface
area. Because of this, as cells become larger, concentrate
flow rate does not increase proportionally to cell volume.
These constraints have led to the use of froth crowders on
virtually all cylindrical machines and to several alternative
launder designs for those machines, as shown in Figure 5.*
Coleman (2009) states that the selection of type, num-
ber, and size of flotation cells required to meet specific
processing needs for an operation depend on the required
flotation residence time and on the amount of concentrate
that can be recovered for a given overflowing lip length
and froth surface area. However, the total lip length and
the surface area at the top of the froth can be modified
by changing the configuration of crowders and launders.
Customizing this configuration is in fact considered a cost-
effective solution for modifying operating parameters such
as froth carry rate and froth lip loading without increasing
the number of flotation tanks.
In large, cylindrical machines, froth recovery is more
difficult for three reasons. First, the distance to a froth laun-
der on the perimeter of the machine is larger. Second, in
large cylindrical cells, the ratio of the surface area at the top
of the cell, through which the froth flows, may not increase
proportionally with increased volume, unless the aspect
ratio of the cylinder is kept constant. Third, the circumfer-
ence of the cell, over which the froth must flow for recovery,
increases as the square root of the volume increase.
MECANICALLY AGITATED CELLS
In the 1970s, as ore grades decreased, mining companies
increased throughput to decrease unit costs. At the same
*A detailed discussion of froth launders can be found in Nelson
and Lelinski 2019.
Figure 5. Left: Metso radial launder with center and peripheral launders (Metso 2024a) Right: FLSmidth radial crowders and
launders (International Mining 2024)