2
still open to investigation in these steps. Additionally, this
paper will highlight some major alternatives which have
been developed to sidestep some of the most difficult prob-
lems of the traditional processes.
LIBERATION
The two fundamental steps of mineral processing are libera-
tion and separation. Comminution implements the libera-
tion step, reducing the size of the rocks to free the different
mineral phases from each other. For the processing of iron-
rich materials into an iron concentrate, the degree of com-
minution required can vary quite a bit, but the majority
of the significant iron ore reserves require fine grinding at
80% passing 25–75 microns to achieve good liberation.
For a typical iron ore concentrator, the comminution
circuit involves autogenous or semi-autogenous mills for
primary grinding, pebble mills used for secondary grind-
ing, and crushers (such cone crushers) used for critical size
materials from the AG/SAG mills (see Jankovic, 2015).
The most typical gangue mineral in iron ore is silica,
notable for being energy intensive to crush and grind. This
results in very considerable grinding energy costs in iron
ore comminution, so there are considerable opportunities
to reduce costs if greater efficiency can be achieved here,
especially in the fine grinding of low-grade ores.
For direct shipping ores, the comminution step is
implemented primarily for convenience of shipping and
handling – the ore is crushed to a desired size target for
lumps, and that is sufficient.
There are some occasions where the comminution step
may be omitted, such as extracting iron from materials like
Bayer process red mud (Archambo and Kawatra, 2021 a,b).
This is typically because the degree of liberation required has
either already been reached or cannot be practically reached
with existing technology. With the Bayer process red mud,
the iron is present in a highly dispersed matrix chemically
precipitated iron-bearing crystals and aluminosilicates. This
material suggests alternative processing routes, as liberation
cannot be reasonably achieved using comminution and
separation cannot be reasonably achieved using existing
mineral processing techniques.
Tailings reprocessing may or may not benefit from
additional liberation – Dauce et al. (2019) suggests the
magnetic concentration of a very coarse iron ore tailings
(80% passing 4mm) with a feed grade of 30.3% Fe which
is already 90% liberated and achieved a final concentrate at
around 50% Fe without additional comminution. On the
other hand, for many concentrators in the U.S. the com-
minution size targets are already 80% passing between 25
and 50 microns. These are pushing the limits of what can
be classified and separated by existing technology, so the
tailings from those processes are comparatively unlikely to
benefit from further comminution either.
Opportunities in Comminution
Achieving particle liberation is vital to support any separa-
tion efforts, but the comminution step is always a primary
energy expenditure of an iron ore concentration plant. The
energy costs of grinding, especially at the fine size ranges
where appropriate liberation is achieved, are very signifi-
cant. Thus, most major opportunities in comminution are
aimed at reducing these energy costs.
One major category of options here are grinding aids,
which are chemicals or materials added to the mills to
improve the grinding process. The primary concern in the
fine grinding of iron ore is the creation of colloidal (e.g., 5
micron) silica slimes. These slimes present two major chal-
lenges in the grinding process:
• They result from overgrinding silica, which is
extremely energy intensive and simultaneously
counterproductive
• They tend to coat the iron-bearing mineral surfaces,
which shields them from further grinding, resulting
in even worse grinding performance
The management of these silica slimes is typically done
utilizing grinding aids. The usefulness of grinding aids has
long been studied in the general case of crushing and grind-
ing (Klimpel and Austin, 1982 Chipakwe et al., 2020).
The application of these technologies to iron ore has gener-
ally found success with simple dispersants such as caustic
soda. The action of caustic soda as a grinding aid follows
two main hypotheses: increasing the pH creates increas-
ingly negative surface potentials on both the iron minerals
and the silica slimes, generating a dispersive effect between
them and that the presence of the sodium hydroxide is
simultaneously effective on dispersing the silica while not
permanently modifying any of the surfaces on the silica or
the iron minerals.
Several other reagents have been tried, but a key restric-
tion for most iron ore processes is that the addition of a
grinding aid cannot interfere with the following flotation
processes. This is a potentially limiting factor for how strong
a dispersant can be for this purpose, as strong dispersants
will compete with the silica for the adsorption of the cat-
ionic collectors typically used for reverse flotation, due to
their own strongly anionic character. Despite this, there are
several potential regions of exploration in this space:
• Moderate strength dispersants which do not overly
collect the flotation collectors
still open to investigation in these steps. Additionally, this
paper will highlight some major alternatives which have
been developed to sidestep some of the most difficult prob-
lems of the traditional processes.
LIBERATION
The two fundamental steps of mineral processing are libera-
tion and separation. Comminution implements the libera-
tion step, reducing the size of the rocks to free the different
mineral phases from each other. For the processing of iron-
rich materials into an iron concentrate, the degree of com-
minution required can vary quite a bit, but the majority
of the significant iron ore reserves require fine grinding at
80% passing 25–75 microns to achieve good liberation.
For a typical iron ore concentrator, the comminution
circuit involves autogenous or semi-autogenous mills for
primary grinding, pebble mills used for secondary grind-
ing, and crushers (such cone crushers) used for critical size
materials from the AG/SAG mills (see Jankovic, 2015).
The most typical gangue mineral in iron ore is silica,
notable for being energy intensive to crush and grind. This
results in very considerable grinding energy costs in iron
ore comminution, so there are considerable opportunities
to reduce costs if greater efficiency can be achieved here,
especially in the fine grinding of low-grade ores.
For direct shipping ores, the comminution step is
implemented primarily for convenience of shipping and
handling – the ore is crushed to a desired size target for
lumps, and that is sufficient.
There are some occasions where the comminution step
may be omitted, such as extracting iron from materials like
Bayer process red mud (Archambo and Kawatra, 2021 a,b).
This is typically because the degree of liberation required has
either already been reached or cannot be practically reached
with existing technology. With the Bayer process red mud,
the iron is present in a highly dispersed matrix chemically
precipitated iron-bearing crystals and aluminosilicates. This
material suggests alternative processing routes, as liberation
cannot be reasonably achieved using comminution and
separation cannot be reasonably achieved using existing
mineral processing techniques.
Tailings reprocessing may or may not benefit from
additional liberation – Dauce et al. (2019) suggests the
magnetic concentration of a very coarse iron ore tailings
(80% passing 4mm) with a feed grade of 30.3% Fe which
is already 90% liberated and achieved a final concentrate at
around 50% Fe without additional comminution. On the
other hand, for many concentrators in the U.S. the com-
minution size targets are already 80% passing between 25
and 50 microns. These are pushing the limits of what can
be classified and separated by existing technology, so the
tailings from those processes are comparatively unlikely to
benefit from further comminution either.
Opportunities in Comminution
Achieving particle liberation is vital to support any separa-
tion efforts, but the comminution step is always a primary
energy expenditure of an iron ore concentration plant. The
energy costs of grinding, especially at the fine size ranges
where appropriate liberation is achieved, are very signifi-
cant. Thus, most major opportunities in comminution are
aimed at reducing these energy costs.
One major category of options here are grinding aids,
which are chemicals or materials added to the mills to
improve the grinding process. The primary concern in the
fine grinding of iron ore is the creation of colloidal (e.g., 5
micron) silica slimes. These slimes present two major chal-
lenges in the grinding process:
• They result from overgrinding silica, which is
extremely energy intensive and simultaneously
counterproductive
• They tend to coat the iron-bearing mineral surfaces,
which shields them from further grinding, resulting
in even worse grinding performance
The management of these silica slimes is typically done
utilizing grinding aids. The usefulness of grinding aids has
long been studied in the general case of crushing and grind-
ing (Klimpel and Austin, 1982 Chipakwe et al., 2020).
The application of these technologies to iron ore has gener-
ally found success with simple dispersants such as caustic
soda. The action of caustic soda as a grinding aid follows
two main hypotheses: increasing the pH creates increas-
ingly negative surface potentials on both the iron minerals
and the silica slimes, generating a dispersive effect between
them and that the presence of the sodium hydroxide is
simultaneously effective on dispersing the silica while not
permanently modifying any of the surfaces on the silica or
the iron minerals.
Several other reagents have been tried, but a key restric-
tion for most iron ore processes is that the addition of a
grinding aid cannot interfere with the following flotation
processes. This is a potentially limiting factor for how strong
a dispersant can be for this purpose, as strong dispersants
will compete with the silica for the adsorption of the cat-
ionic collectors typically used for reverse flotation, due to
their own strongly anionic character. Despite this, there are
several potential regions of exploration in this space:
• Moderate strength dispersants which do not overly
collect the flotation collectors