294 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
to a storage tank before being re-looped in the carbonation
unit for future reaction (Figure 1). Also, a processing unit
with sonication will remove the passivating layer after car-
bonation and help achieve higher serpentine conversion
to magnesite by allowing multiple passes of unconverted
serpentine through the carbonation unit. The carbonated
slurry would then be sent to the froth flotation column,
where the nickel-bearing pentlandite can be separated from
the magnesium carbonate tails. In addition to an enhanced
nickel concentrate flotation performance, the formed mag-
nesite (MgCO3) in the tailings has the potential as a com-
mercial reagent.
While discussing the mineral carbonation of ultramafic
sulfide ore as a technique to sequester CO2 emissions, it is
important to discuss the role that CO2 solubility and dis-
solved divalent oxides (such as Mg2+, Ca2+ and Fe2+) play
in accelerating carbonation reaction.
Role of Solubility of CO2 in Water
CO2 solubility is an important factor as the carbonic acid
formation is the rate-determining step that defines the
kinetics for enhanced carbonation conversion (O’Connor
et al., 2005). The CO2 solubility limits the reaction rate,
and therefore, the reaction requires higher CO2 dissolution
for it to be available to form carbonates. The solubility of
CO2 increases with pressure up to approximately 30 MPa
(~300 atm), above which there is an optimum solubility in
the range of 60–70 °C (Perkins, 2003). At 1 atm and tem-
peratures in the 20–40 °C range (expected conditions at
industrial scale), the solubility of CO2 is negligible (2 kg
CO2/t H2O), and significant improvements in CO2 sol-
ubility can be achieved by ramping up the pressure. For
instance, increasing the conditions to 100 atm and 60 °C
would see CO2 solubility increase to approximately 45 kg
CO2/t H2O, an increase of 800–1000 times the solubil-
ity. Consequently, carbonation reactions are likely far more
efficient at higher pressures.
Role of Oxides for Carbonation
The presence of Mg, Ca, or Fe oxides is essential for effec-
tive carbonation. In the case of serpentines, MgO reacts
with dissolved CO2 to form a stable magnesium carbonate,
magnesite (MgCO3) (Eqns. 5–6). A pre-treatment or acti-
vation step is usually employed to efficiently dissolve the
Mg from serpentine minerals. As a result, research on min-
eral carbonation has focused on efficient mineral activation,
determining the optimum conditions for mineral dissolu-
tion and precipitation, and enhancing reaction kinetics to
maximize conversion (Chizmeshya et al., 2007 Pasquier et
al., 2014 Tebbiche et al., 2021).
Nickel Tailings Carbonation
The serpentine tailings carbonation is similar to processing
the ultramafic nickel sulfide in terms of reaction mecha-
nism (Eqns. 1–6), except that here the tailings having ser-
pentine are carbonated instead of fresh ore feed (Figure 2).
This provides more flexibility in tailings carbonation as they
are the final product and thus do not impede the processing
speed of other sub-processes, i.e., they can be carbonated at
optimized reaction conditions and for a longer duration.
A recent pilot-scale tailings carbonation feasibility study
by Canada Nickel on In-Process Tailings (IPT) reveals that
it can securely contain 1 million tonnes of CO2 annually,
with a 6.5-hour residence time in the expected tailings from
the upcoming Crawford nickel sulfide project. The cost of
this storage comes at around CA$25/ t CO2. Furthermore,
tailings mineralization positions Canada Nickel for carbon
Figure 1. High-level block diagram for carbonation-assisted
froth flotation of nickel sulfide ore
to a storage tank before being re-looped in the carbonation
unit for future reaction (Figure 1). Also, a processing unit
with sonication will remove the passivating layer after car-
bonation and help achieve higher serpentine conversion
to magnesite by allowing multiple passes of unconverted
serpentine through the carbonation unit. The carbonated
slurry would then be sent to the froth flotation column,
where the nickel-bearing pentlandite can be separated from
the magnesium carbonate tails. In addition to an enhanced
nickel concentrate flotation performance, the formed mag-
nesite (MgCO3) in the tailings has the potential as a com-
mercial reagent.
While discussing the mineral carbonation of ultramafic
sulfide ore as a technique to sequester CO2 emissions, it is
important to discuss the role that CO2 solubility and dis-
solved divalent oxides (such as Mg2+, Ca2+ and Fe2+) play
in accelerating carbonation reaction.
Role of Solubility of CO2 in Water
CO2 solubility is an important factor as the carbonic acid
formation is the rate-determining step that defines the
kinetics for enhanced carbonation conversion (O’Connor
et al., 2005). The CO2 solubility limits the reaction rate,
and therefore, the reaction requires higher CO2 dissolution
for it to be available to form carbonates. The solubility of
CO2 increases with pressure up to approximately 30 MPa
(~300 atm), above which there is an optimum solubility in
the range of 60–70 °C (Perkins, 2003). At 1 atm and tem-
peratures in the 20–40 °C range (expected conditions at
industrial scale), the solubility of CO2 is negligible (2 kg
CO2/t H2O), and significant improvements in CO2 sol-
ubility can be achieved by ramping up the pressure. For
instance, increasing the conditions to 100 atm and 60 °C
would see CO2 solubility increase to approximately 45 kg
CO2/t H2O, an increase of 800–1000 times the solubil-
ity. Consequently, carbonation reactions are likely far more
efficient at higher pressures.
Role of Oxides for Carbonation
The presence of Mg, Ca, or Fe oxides is essential for effec-
tive carbonation. In the case of serpentines, MgO reacts
with dissolved CO2 to form a stable magnesium carbonate,
magnesite (MgCO3) (Eqns. 5–6). A pre-treatment or acti-
vation step is usually employed to efficiently dissolve the
Mg from serpentine minerals. As a result, research on min-
eral carbonation has focused on efficient mineral activation,
determining the optimum conditions for mineral dissolu-
tion and precipitation, and enhancing reaction kinetics to
maximize conversion (Chizmeshya et al., 2007 Pasquier et
al., 2014 Tebbiche et al., 2021).
Nickel Tailings Carbonation
The serpentine tailings carbonation is similar to processing
the ultramafic nickel sulfide in terms of reaction mecha-
nism (Eqns. 1–6), except that here the tailings having ser-
pentine are carbonated instead of fresh ore feed (Figure 2).
This provides more flexibility in tailings carbonation as they
are the final product and thus do not impede the processing
speed of other sub-processes, i.e., they can be carbonated at
optimized reaction conditions and for a longer duration.
A recent pilot-scale tailings carbonation feasibility study
by Canada Nickel on In-Process Tailings (IPT) reveals that
it can securely contain 1 million tonnes of CO2 annually,
with a 6.5-hour residence time in the expected tailings from
the upcoming Crawford nickel sulfide project. The cost of
this storage comes at around CA$25/ t CO2. Furthermore,
tailings mineralization positions Canada Nickel for carbon
Figure 1. High-level block diagram for carbonation-assisted
froth flotation of nickel sulfide ore