XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2671
CASE 2: CALCIUM AND MAGNESIUM
Calcium and magnesium are very common ions in process
water, typically found as major components of water hard-
ness. Calcium and magnesium are particularly important
because they both demonstrate strong adsorption onto iron
ore surfaces and can interact with many of the reagents
which are used in iron ore flotation. Calcium and magne-
sium both exist as divalent cations (Ca2+/Mg2+) in process
water, which are capable of adsorption onto negatively
charged mineral surfaces. In the reverse flotation process
that has been mentioned before, this includes some adsorp-
tion onto the negatively charged silica surfaces, but calcium
and magnesium are known to strongly associate with nega-
tively charged hematite/magnetite surfaces.
Calcium and magnesium are concentrated in the iron
ore throughout the reverse flotation process, with filter
cakes achieving concentrations of 5,000 to 6,000 parts per
million at some operating plants even with relatively soft
water for process water (Ripke and Kawatra, 2003 Kawatra
and Claremboux, 2019). This concentration on the surfaces
of the iron ore minerals means that even small quantities of
these ions in the process water can have significant effects
during the separation process and downstream in filtering
or agglomeration.
The main impact of calcium and magnesium is two-
fold: one, they strongly influence the ionic strength of the
water, which has considerable influence on the water chem-
istry and the effects of surface charges on the behavior of
the system and two, they directly affect the surface charges
of the iron-bearing minerals as specifically adsorbed ions.
These ions are known to have an overarching impact on
most aspects of the concentration process.
The presence of more than 15ppm of calcium and
magnesium combined is highly detrimental to the des-
lime process (Green and Colombo, 1984 Haselhuhn and
Kawatra, 2015). Later works have shown the presence of a
small amount of calcium ions is beneficial to flotation, but
any dissolved magnesium and large amounts of calcium are
detrimental again (Parra-Álvarez et al., 2023).
As specifically adsorbed ions calcium and magnesium
both strongly adsorb to the negatively charged surfaces of
iron ore minerals and surface charges to become less nega-
tively charged, making them neutral or positively charged.
However, the reverse cationic flotation process targets a
highly alkaline pH of around 10.5–11.5 (around pH 11
for the process in Figure 1) specifically to ensure nega-
tive surface charges on all surfaces in the flotation. These
negative surface charges are a critical part of ensuring that
the depressants and collectors used in the flotation attach
to the right species and allow proper separation of the
iron-bearing minerals from the gangue. The typical action
of excess calcium or magnesium is to depress both species
reducing separation efficiency and selectivity (Lelis et al.,
2019 Da Cruz et al., 2021).
For the same reason, these ions tend to interact strongly
with anionic species such as dispersants or causticized
starch. The relatively strong positive charges on both cal-
cium and magnesium result in potent interactions between
the two species. The impact of magnesium is more extreme
due to its smaller ionic radius, so even though it has the
same charge as calcium, magnesium exhibits a higher
charge density (Parra-Álvarez et al., 2023).
These species are also considerably important in anionic
flotation. Calcium and magnesium can serve as activators
for anionic collectors. The stronger impact of magnesium
has been exploited in at least one plant to reduce the pH
necessary to achieve proper activation of the silica in fatty
acid flotation, but it was also noted to be more selective
towards specifically quartz gangues (Sandvik and Larsen,
2014).
Lelis et al. (2019) found that the effect of calcium on
hematite (in reverse cationic flotation of itabaritic ores) and
silica was similar, resulting in deactivation of both minerals
at high concentrations. They also found that the subsequent
removal of the calcium by the addition of ethylenediamine-
tetraacetic acid (EDTA) resulted in the reactivation of the
quartz but not the hematite.
Notably, selective flocculation and dispersion deslim-
ing is quite dependent on both dispersants and starch. The
potential extent of the impact on desliming can be seen in
trialing different dispersants in general, where a difference
in iron recovery of up to 4.6% between different dispersant
choices was observed by the authors at a plant trial.
Calcium and magnesium are most strongly concen-
trated into the filtered iron ore concentrate, and which sub-
sequently impacts the effectiveness of pelletization (Kawatra
and Ripke, 2003 Ripke and Kawatra, 2003 Eisele et al.,
2005). At one plant an attempt was made to remove these
ions during the filtering step by adding CO2 to lower the
pH and preferably desorb the ions from the filter cake. The
CO2 mixed with the filtrate water to lower the pH and thus
make the surface charge of the filtered solids more positive
(Haselhuhn et al., 2012a Kawatra and Claremboux, 2019).
The intention was that the more positive surfaces would
repel and desorb the calcium and magnesium cations.
The added CO2 had a noticeable impact on the pH
and the zeta potential of the hematite filter cake (Carlson
and Kawatra, 2011). The tendency of the hematite to dis-
perse itself was thusly reduced accordingly. However, at the
CASE 2: CALCIUM AND MAGNESIUM
Calcium and magnesium are very common ions in process
water, typically found as major components of water hard-
ness. Calcium and magnesium are particularly important
because they both demonstrate strong adsorption onto iron
ore surfaces and can interact with many of the reagents
which are used in iron ore flotation. Calcium and magne-
sium both exist as divalent cations (Ca2+/Mg2+) in process
water, which are capable of adsorption onto negatively
charged mineral surfaces. In the reverse flotation process
that has been mentioned before, this includes some adsorp-
tion onto the negatively charged silica surfaces, but calcium
and magnesium are known to strongly associate with nega-
tively charged hematite/magnetite surfaces.
Calcium and magnesium are concentrated in the iron
ore throughout the reverse flotation process, with filter
cakes achieving concentrations of 5,000 to 6,000 parts per
million at some operating plants even with relatively soft
water for process water (Ripke and Kawatra, 2003 Kawatra
and Claremboux, 2019). This concentration on the surfaces
of the iron ore minerals means that even small quantities of
these ions in the process water can have significant effects
during the separation process and downstream in filtering
or agglomeration.
The main impact of calcium and magnesium is two-
fold: one, they strongly influence the ionic strength of the
water, which has considerable influence on the water chem-
istry and the effects of surface charges on the behavior of
the system and two, they directly affect the surface charges
of the iron-bearing minerals as specifically adsorbed ions.
These ions are known to have an overarching impact on
most aspects of the concentration process.
The presence of more than 15ppm of calcium and
magnesium combined is highly detrimental to the des-
lime process (Green and Colombo, 1984 Haselhuhn and
Kawatra, 2015). Later works have shown the presence of a
small amount of calcium ions is beneficial to flotation, but
any dissolved magnesium and large amounts of calcium are
detrimental again (Parra-Álvarez et al., 2023).
As specifically adsorbed ions calcium and magnesium
both strongly adsorb to the negatively charged surfaces of
iron ore minerals and surface charges to become less nega-
tively charged, making them neutral or positively charged.
However, the reverse cationic flotation process targets a
highly alkaline pH of around 10.5–11.5 (around pH 11
for the process in Figure 1) specifically to ensure nega-
tive surface charges on all surfaces in the flotation. These
negative surface charges are a critical part of ensuring that
the depressants and collectors used in the flotation attach
to the right species and allow proper separation of the
iron-bearing minerals from the gangue. The typical action
of excess calcium or magnesium is to depress both species
reducing separation efficiency and selectivity (Lelis et al.,
2019 Da Cruz et al., 2021).
For the same reason, these ions tend to interact strongly
with anionic species such as dispersants or causticized
starch. The relatively strong positive charges on both cal-
cium and magnesium result in potent interactions between
the two species. The impact of magnesium is more extreme
due to its smaller ionic radius, so even though it has the
same charge as calcium, magnesium exhibits a higher
charge density (Parra-Álvarez et al., 2023).
These species are also considerably important in anionic
flotation. Calcium and magnesium can serve as activators
for anionic collectors. The stronger impact of magnesium
has been exploited in at least one plant to reduce the pH
necessary to achieve proper activation of the silica in fatty
acid flotation, but it was also noted to be more selective
towards specifically quartz gangues (Sandvik and Larsen,
2014).
Lelis et al. (2019) found that the effect of calcium on
hematite (in reverse cationic flotation of itabaritic ores) and
silica was similar, resulting in deactivation of both minerals
at high concentrations. They also found that the subsequent
removal of the calcium by the addition of ethylenediamine-
tetraacetic acid (EDTA) resulted in the reactivation of the
quartz but not the hematite.
Notably, selective flocculation and dispersion deslim-
ing is quite dependent on both dispersants and starch. The
potential extent of the impact on desliming can be seen in
trialing different dispersants in general, where a difference
in iron recovery of up to 4.6% between different dispersant
choices was observed by the authors at a plant trial.
Calcium and magnesium are most strongly concen-
trated into the filtered iron ore concentrate, and which sub-
sequently impacts the effectiveness of pelletization (Kawatra
and Ripke, 2003 Ripke and Kawatra, 2003 Eisele et al.,
2005). At one plant an attempt was made to remove these
ions during the filtering step by adding CO2 to lower the
pH and preferably desorb the ions from the filter cake. The
CO2 mixed with the filtrate water to lower the pH and thus
make the surface charge of the filtered solids more positive
(Haselhuhn et al., 2012a Kawatra and Claremboux, 2019).
The intention was that the more positive surfaces would
repel and desorb the calcium and magnesium cations.
The added CO2 had a noticeable impact on the pH
and the zeta potential of the hematite filter cake (Carlson
and Kawatra, 2011). The tendency of the hematite to dis-
perse itself was thusly reduced accordingly. However, at the