3580 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
saleable concentrates containing at least 65% iron content.
The beneficiation of taconite involves crushing, grinding,
classification, concentration using physical beneficiation
and flotation. The process route depends on the taconite
mineralogy and product specification.
Comminution Circuit
The comminution circuits for taconite processing employ
gyratory and cone crushers for primary and secondary
crushing stages, respectively, with some operations having
as many as four stages of crushing when using rod milling.
Crusher products are classified using vibrating screens, with
the screen oversize material recycled back to the crushers.
The crusher circuit product is then fed into rod and ball
mills for further size reduction. Some operations, specifi-
cally on the western Mesabi in Minnesota (e.g., Keewatin
Taconite and Hibbing Taconite) and Tilden Mine in
Michigan, employ semi-autogenous (SAG) or autogenous
(AG) grinding mills for the grinding duty. The mills are
generally operated in closed circuits with hydroclones for
grind control, but some of the operations (e.g., Keewatin
Taconite) have transitioned to fine screening, utilizing
Derrick Stack Sizers.
Concentration
Concentration processes for low-grade magnetic and hema-
titic taconite were developed in Minnesota in the 1950s
and 1960s (Poveromo, 1999). Taconites are concentrated
using magnetic separation, hydroseparation and some-
times reverse cationic flotation, specifically in the central
and eastern portions of the Mesabi Iron Formation in
Minnesota. Magnetic separation involves multiple stages
to produce a concentrate, with non-magnetic gangue par-
ticles discarded as waste. For magnetic taconite, low inten-
sity magnetic separation (LIMS) is employed to produce a
magnetic concentrate, usually grading at 4.5–6% silica. The
magnetic concentrate is sometimes reground in a regrind
mill prior reverse cationic flotation (RCF), especially when
generating low-silica (2% SiO2) concentrate. RCF is
employed to clean up the magnetite concentrate to produce
blast furnace (BF) grade concentrates (4.5–5.5% SiO2) or
low-silica direct reduced (DR) concentrates (2% silica).
RCF is conducted at a neutral pH of 8, using amine as col-
lector and methyl isobutyl carbinol (MIBC) as a frother.
For hematite ores, RCF is employed as the primary
recovery method for the iron bearing minerals. Selective
flocculation, developed by the U.S. Bureau of Mines and
Cleveland-Cliffs, utilized at the Tilden plant in Michigan,
is used to remove siliceous slimes (Poveromo, 1999). The
selective flocculation process is sensitive to water hardness
(Ca2+ +Mg2+), and hence process water is pre-treated to
reduce water hardness by precipitating Ca2+ and Mg2+ ions
using sodium hydroxide (Iwasaki, 2020). Selective floccu-
lation is conducted at an alkaline pH of 11–12, sodium
silicate is used as a dispersant, polysaccharide as a flocculent
and starch as a hematite depressant. The pH of the flota-
tion feed is then adjusted to a neutral pH of 8 using CO2
gas injection prior RCF. An amine is used as a silica collec-
tor and MIBC as a frother during RCF. Wet high inten-
sity magnetic separation (WHIMS) has been employed
to produce a magnetic concentrate from tailings ponds in
Minnesota. Table 2 presents details about different flow-
sheet configurations currently employed by different active
iron ore mines in the USA.
CHALLENGES FACING THE U.S. IRON
ORE INDUSTRY
The major challenges facing the USA iron ore mining
industry include:
1. Declining cut-off grades.
2. High energy consumption because of the require-
ment for fine grinding to 25–44 µm to enhance
the liberation of Fe-bearing minerals.
3. Loss of non-magnetic iron minerals in magnetic
concentrators.
4. Loss of fine hematite in RCF concentrators due to
entrainment.
5. Stringent sulphate regulation of 10 ppm.
6. Competition from international iron and steel
making players.
Figure 2. Typical flow sheet for high-grade ore for producing
DSO (Nunna, 2022)
saleable concentrates containing at least 65% iron content.
The beneficiation of taconite involves crushing, grinding,
classification, concentration using physical beneficiation
and flotation. The process route depends on the taconite
mineralogy and product specification.
Comminution Circuit
The comminution circuits for taconite processing employ
gyratory and cone crushers for primary and secondary
crushing stages, respectively, with some operations having
as many as four stages of crushing when using rod milling.
Crusher products are classified using vibrating screens, with
the screen oversize material recycled back to the crushers.
The crusher circuit product is then fed into rod and ball
mills for further size reduction. Some operations, specifi-
cally on the western Mesabi in Minnesota (e.g., Keewatin
Taconite and Hibbing Taconite) and Tilden Mine in
Michigan, employ semi-autogenous (SAG) or autogenous
(AG) grinding mills for the grinding duty. The mills are
generally operated in closed circuits with hydroclones for
grind control, but some of the operations (e.g., Keewatin
Taconite) have transitioned to fine screening, utilizing
Derrick Stack Sizers.
Concentration
Concentration processes for low-grade magnetic and hema-
titic taconite were developed in Minnesota in the 1950s
and 1960s (Poveromo, 1999). Taconites are concentrated
using magnetic separation, hydroseparation and some-
times reverse cationic flotation, specifically in the central
and eastern portions of the Mesabi Iron Formation in
Minnesota. Magnetic separation involves multiple stages
to produce a concentrate, with non-magnetic gangue par-
ticles discarded as waste. For magnetic taconite, low inten-
sity magnetic separation (LIMS) is employed to produce a
magnetic concentrate, usually grading at 4.5–6% silica. The
magnetic concentrate is sometimes reground in a regrind
mill prior reverse cationic flotation (RCF), especially when
generating low-silica (2% SiO2) concentrate. RCF is
employed to clean up the magnetite concentrate to produce
blast furnace (BF) grade concentrates (4.5–5.5% SiO2) or
low-silica direct reduced (DR) concentrates (2% silica).
RCF is conducted at a neutral pH of 8, using amine as col-
lector and methyl isobutyl carbinol (MIBC) as a frother.
For hematite ores, RCF is employed as the primary
recovery method for the iron bearing minerals. Selective
flocculation, developed by the U.S. Bureau of Mines and
Cleveland-Cliffs, utilized at the Tilden plant in Michigan,
is used to remove siliceous slimes (Poveromo, 1999). The
selective flocculation process is sensitive to water hardness
(Ca2+ +Mg2+), and hence process water is pre-treated to
reduce water hardness by precipitating Ca2+ and Mg2+ ions
using sodium hydroxide (Iwasaki, 2020). Selective floccu-
lation is conducted at an alkaline pH of 11–12, sodium
silicate is used as a dispersant, polysaccharide as a flocculent
and starch as a hematite depressant. The pH of the flota-
tion feed is then adjusted to a neutral pH of 8 using CO2
gas injection prior RCF. An amine is used as a silica collec-
tor and MIBC as a frother during RCF. Wet high inten-
sity magnetic separation (WHIMS) has been employed
to produce a magnetic concentrate from tailings ponds in
Minnesota. Table 2 presents details about different flow-
sheet configurations currently employed by different active
iron ore mines in the USA.
CHALLENGES FACING THE U.S. IRON
ORE INDUSTRY
The major challenges facing the USA iron ore mining
industry include:
1. Declining cut-off grades.
2. High energy consumption because of the require-
ment for fine grinding to 25–44 µm to enhance
the liberation of Fe-bearing minerals.
3. Loss of non-magnetic iron minerals in magnetic
concentrators.
4. Loss of fine hematite in RCF concentrators due to
entrainment.
5. Stringent sulphate regulation of 10 ppm.
6. Competition from international iron and steel
making players.
Figure 2. Typical flow sheet for high-grade ore for producing
DSO (Nunna, 2022)