XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 165
this analysis is comminution flowsheet consideration based
on ore competency and hardness values as well as media
consumption and grind size. Insights regarding feed source,
grind size (i.e., surface area) and mineralogical composition
are also provided in the context of defining a suitable com-
minution flowsheet that maximizes CO2 absorption.
MINERAL CARBONATION
Olivine is the collective term for a silicate solid solu-
tion series ranging from fayalite (Fe2SiO4) to forsterite
(Mg2SiO4). The carbonation of forsterite to yield magne-
sium carbonate (magnesite) and silica is described in equa-
tions 1 to 3 (Haug, 2010).
CO2 +H2O ⇌ H2CO3 ⇌ HCO3– +H+
⇌ CO32– +H+ (1)
Mg2SiO4 +4 H+ ⇌ 2 Mg2+ +H4SiO4 (2)
Mg2+ +CO32– ⇌ MgCO3 (3)
Considering that water has a catalytic function in this reac-
tion, this gives the following net aqueous carbonation reac-
tion (4) (Haug, 2010):
Mg2SiO4 +2 CO2 → 2 MgCO3 +SiO2 (4)
This means that stoichiometrically, for every forsterite mol-
ecule two CO2 molecules can be reacted. Based on the
molar masses of the reactants, this equates to a maximum
theoretical CO2 sequestration capacity of 0.63 t CO2 per
tonne of forsterite. Aside from technical limitations, this
theoretical maximum is unlikely to be achieved because of
the fayalite-forsterite solid solution series (Fe to Mg oliv-
ine) and other impurities in the rock reducing the reactable
(Mg) content. Figure 1 illustrates the theoretical maximum
CO2 sequestration capacity based on forsterite content,
stoichiometry, the net carbonation reaction (equation 4)
and puts this in the perspective of different ultramafic rock
types. Realistically, 95–98% of the theoretical CO2 seques-
tration capacity of olivine would be a good practical maxi-
mum carbon sequestration limit for dunites.
Gerdemann et al. (2007) showed the optimum con-
ditions for olivine carbonation to be around 185 °C and
150 atm PCO2, showing this reaction requires quite extreme
conditions for favourable reaction kinetics. The addition
of 0.64M NaHCO3 increases ionic strength of the solu-
tion and aids dissolution of Si, ultimately preventing for-
mation of a Si-rich passifying layer and addition of 1M
NaCl is also commonly quoted as aiding the carbonation
reaction (O’Connor et al., 2005 Gerdemann et al., 2007
Wang et al., 2019). Other minerals such as serpentine
(Mg3Si2O5(OH)4) and wollastonite (CaSiO3) can also
be reacted but the majority of research is done on olivine.
These minerals require less extreme conditions (e.g., wollas-
tonite optimum carbonation at 100 °C and 40 atm PCO2
Figure 1. Theoretical CO2 sequestration capacity as a function of forsterite content
this analysis is comminution flowsheet consideration based
on ore competency and hardness values as well as media
consumption and grind size. Insights regarding feed source,
grind size (i.e., surface area) and mineralogical composition
are also provided in the context of defining a suitable com-
minution flowsheet that maximizes CO2 absorption.
MINERAL CARBONATION
Olivine is the collective term for a silicate solid solu-
tion series ranging from fayalite (Fe2SiO4) to forsterite
(Mg2SiO4). The carbonation of forsterite to yield magne-
sium carbonate (magnesite) and silica is described in equa-
tions 1 to 3 (Haug, 2010).
CO2 +H2O ⇌ H2CO3 ⇌ HCO3– +H+
⇌ CO32– +H+ (1)
Mg2SiO4 +4 H+ ⇌ 2 Mg2+ +H4SiO4 (2)
Mg2+ +CO32– ⇌ MgCO3 (3)
Considering that water has a catalytic function in this reac-
tion, this gives the following net aqueous carbonation reac-
tion (4) (Haug, 2010):
Mg2SiO4 +2 CO2 → 2 MgCO3 +SiO2 (4)
This means that stoichiometrically, for every forsterite mol-
ecule two CO2 molecules can be reacted. Based on the
molar masses of the reactants, this equates to a maximum
theoretical CO2 sequestration capacity of 0.63 t CO2 per
tonne of forsterite. Aside from technical limitations, this
theoretical maximum is unlikely to be achieved because of
the fayalite-forsterite solid solution series (Fe to Mg oliv-
ine) and other impurities in the rock reducing the reactable
(Mg) content. Figure 1 illustrates the theoretical maximum
CO2 sequestration capacity based on forsterite content,
stoichiometry, the net carbonation reaction (equation 4)
and puts this in the perspective of different ultramafic rock
types. Realistically, 95–98% of the theoretical CO2 seques-
tration capacity of olivine would be a good practical maxi-
mum carbon sequestration limit for dunites.
Gerdemann et al. (2007) showed the optimum con-
ditions for olivine carbonation to be around 185 °C and
150 atm PCO2, showing this reaction requires quite extreme
conditions for favourable reaction kinetics. The addition
of 0.64M NaHCO3 increases ionic strength of the solu-
tion and aids dissolution of Si, ultimately preventing for-
mation of a Si-rich passifying layer and addition of 1M
NaCl is also commonly quoted as aiding the carbonation
reaction (O’Connor et al., 2005 Gerdemann et al., 2007
Wang et al., 2019). Other minerals such as serpentine
(Mg3Si2O5(OH)4) and wollastonite (CaSiO3) can also
be reacted but the majority of research is done on olivine.
These minerals require less extreme conditions (e.g., wollas-
tonite optimum carbonation at 100 °C and 40 atm PCO2
Figure 1. Theoretical CO2 sequestration capacity as a function of forsterite content