Production of Mg(OH)2 from Mg-silicate rock for CO2 mineral sequestration
Nduagu, Experience (2012-12-13)
Nduagu, Experience
Åbo Akademi - Åbo Akademi University
13.12.2012
Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe201311117327
https://urn.fi/URN:NBN:fi-fe201311117327
Tiivistelmä
Sequestration of carbon dioxide in mineral rocks, also known as CO2 Capture and Mineralization
(CCM), is considered to have a huge potential in stabilizing anthropogenic CO2 emissions. One
of the CCM routes is the ex situ indirect gas/sold carbonation of reactive materials, such as
Mg(OH)2, produced from abundantly available Mg-silicate rocks. The gas/solid carbonation
method is intensively researched at Åbo Akademi University (ÅAU ), Finland because it is
energetically attractive and utilizes the exothermic chemistry of Mg(OH)2 carbonation. In this
thesis, a method for producing Mg(OH)2 from Mg-silicate rocks for CCM was investigated, and
the process efficiency, energy and environmental impact assessed. The Mg(OH)2 process studied
here was first proposed in 2008 in a Master’s Thesis by the author. At that time the process was
applied to only one Mg-silicate rock (Finnish serpentinite from the Hitura nickel mine site of
Finn Nickel) and the optimum process conversions, energy and environmental performance
were not known.
Producing Mg(OH)2 from Mg-silicate rocks involves a two-staged process of Mg extraction and
Mg(OH)2 precipitation. The first stage extracts Mg and other cations by reacting pulverized
serpentinite or olivine rocks with ammonium sulfate (AS) salt at 400 - 550 oC (preferably < 450
oC). In the second stage, ammonia solution reacts with the cations (extracted from the first stage
after they are leached in water) to form mainly FeOOH, high purity Mg(OH)2 and aqueous
(dissolved) AS. The Mg(OH)2 process described here is closed loop in nature; gaseous ammonia
and water vapour are produced from the extraction stage, recovered and used as reagent for the
precipitation stage. The AS reagent is thereafter recovered after the precipitation stage.
The Mg extraction stage, being the conversion-determining and the most energy-intensive step
of the entire CCM process chain, received a prominent attention in this study. The extraction
behavior and reactivity of different rocks types (serpentinite and olivine rocks) from different
locations worldwide (Australia, Finland, Lithuania, Norway and Portugal) was tested. Also,
parametric evaluation was carried out to determine the optimal reaction temperature, time and
chemical reagent (AS). Effects of reactor types and configuration, mixing and scale-up
possibilities were also studied. The Mg(OH)2 produced can be used to convert CO2 to
thermodynamically stable and environmentally benign magnesium carbonate. Therefore, the
process energy and life cycle environmental performance of the ÅAU CCM technique that first
produces Mg(OH)2 and the carbonates in a pressurized fluidized bed (FB) were assessed. The life
cycle energy and environmental assessment approach applied in this thesis is motivated by the
fact that the CCM technology should in itself offer a solution to what is both an energy and
environmental problem.
Results obtained in this study show that different Mg-silicate rocks react differently; olivine rocks
being far less reactive than serpentinite rocks. In summary, the reactivity of Mg-silicate rocks is a
function of both the chemical and physical properties of rocks. Reaction temperature and time
remain important parameters to consider in process design and operation. Heat transfer
properties of the reactor determine the temperature at which maximum Mg extraction is
obtained. Also, an increase in reaction temperature leads to an increase in the extent of
extraction, reaching a maximum yield at different temperatures depending on the reaction time.
Process energy requirement for producing Mg(OH)2 from a hypothetical case of an iron-free
serpentine rock is 3.62 GJ/t-CO2. This value can increase by 16 - 68% depending on the type of
iron compound (FeO, Fe2O3 or Fe3O4) in the mineral. This suggests that the benefit from the
potential use of FeOOH as an iron ore feedstock in iron and steelmaking should be determined
by considering the energy, cost and emissions associated with the FeOOH by-product. AS
recovery through crystallization is the second most energy intensive unit operation after the
extraction reaction. However, the choice of mechanical vapor recompression (MVR) over the
“simple evaporation” crystallization method has a potential energy savings of 15.2 GJ/t-CO2 (84
% savings). Integrating the Mg(OH)2 production method and the gas/solid carbonation process
could provide up to an 25% energy offset to the CCM process energy requirements. Life cycle
inventory assessment (LCIA) results show that for every ton of CO2 mineralized, the ÅAU CCM
process avoids 430 - 480 kg CO2.
The Mg(OH)2 process studied in this thesis has many promising features. Even at the current
high energy and environmental burden, producing Mg(OH)2 from Mg-silicates can play a
significant role in advancing CCM processes. However, dedicated future research and
development (R&D) have potential to significantly improve the Mg(OH)2 process performance.
(CCM), is considered to have a huge potential in stabilizing anthropogenic CO2 emissions. One
of the CCM routes is the ex situ indirect gas/sold carbonation of reactive materials, such as
Mg(OH)2, produced from abundantly available Mg-silicate rocks. The gas/solid carbonation
method is intensively researched at Åbo Akademi University (ÅAU ), Finland because it is
energetically attractive and utilizes the exothermic chemistry of Mg(OH)2 carbonation. In this
thesis, a method for producing Mg(OH)2 from Mg-silicate rocks for CCM was investigated, and
the process efficiency, energy and environmental impact assessed. The Mg(OH)2 process studied
here was first proposed in 2008 in a Master’s Thesis by the author. At that time the process was
applied to only one Mg-silicate rock (Finnish serpentinite from the Hitura nickel mine site of
Finn Nickel) and the optimum process conversions, energy and environmental performance
were not known.
Producing Mg(OH)2 from Mg-silicate rocks involves a two-staged process of Mg extraction and
Mg(OH)2 precipitation. The first stage extracts Mg and other cations by reacting pulverized
serpentinite or olivine rocks with ammonium sulfate (AS) salt at 400 - 550 oC (preferably < 450
oC). In the second stage, ammonia solution reacts with the cations (extracted from the first stage
after they are leached in water) to form mainly FeOOH, high purity Mg(OH)2 and aqueous
(dissolved) AS. The Mg(OH)2 process described here is closed loop in nature; gaseous ammonia
and water vapour are produced from the extraction stage, recovered and used as reagent for the
precipitation stage. The AS reagent is thereafter recovered after the precipitation stage.
The Mg extraction stage, being the conversion-determining and the most energy-intensive step
of the entire CCM process chain, received a prominent attention in this study. The extraction
behavior and reactivity of different rocks types (serpentinite and olivine rocks) from different
locations worldwide (Australia, Finland, Lithuania, Norway and Portugal) was tested. Also,
parametric evaluation was carried out to determine the optimal reaction temperature, time and
chemical reagent (AS). Effects of reactor types and configuration, mixing and scale-up
possibilities were also studied. The Mg(OH)2 produced can be used to convert CO2 to
thermodynamically stable and environmentally benign magnesium carbonate. Therefore, the
process energy and life cycle environmental performance of the ÅAU CCM technique that first
produces Mg(OH)2 and the carbonates in a pressurized fluidized bed (FB) were assessed. The life
cycle energy and environmental assessment approach applied in this thesis is motivated by the
fact that the CCM technology should in itself offer a solution to what is both an energy and
environmental problem.
Results obtained in this study show that different Mg-silicate rocks react differently; olivine rocks
being far less reactive than serpentinite rocks. In summary, the reactivity of Mg-silicate rocks is a
function of both the chemical and physical properties of rocks. Reaction temperature and time
remain important parameters to consider in process design and operation. Heat transfer
properties of the reactor determine the temperature at which maximum Mg extraction is
obtained. Also, an increase in reaction temperature leads to an increase in the extent of
extraction, reaching a maximum yield at different temperatures depending on the reaction time.
Process energy requirement for producing Mg(OH)2 from a hypothetical case of an iron-free
serpentine rock is 3.62 GJ/t-CO2. This value can increase by 16 - 68% depending on the type of
iron compound (FeO, Fe2O3 or Fe3O4) in the mineral. This suggests that the benefit from the
potential use of FeOOH as an iron ore feedstock in iron and steelmaking should be determined
by considering the energy, cost and emissions associated with the FeOOH by-product. AS
recovery through crystallization is the second most energy intensive unit operation after the
extraction reaction. However, the choice of mechanical vapor recompression (MVR) over the
“simple evaporation” crystallization method has a potential energy savings of 15.2 GJ/t-CO2 (84
% savings). Integrating the Mg(OH)2 production method and the gas/solid carbonation process
could provide up to an 25% energy offset to the CCM process energy requirements. Life cycle
inventory assessment (LCIA) results show that for every ton of CO2 mineralized, the ÅAU CCM
process avoids 430 - 480 kg CO2.
The Mg(OH)2 process studied in this thesis has many promising features. Even at the current
high energy and environmental burden, producing Mg(OH)2 from Mg-silicates can play a
significant role in advancing CCM processes. However, dedicated future research and
development (R&D) have potential to significantly improve the Mg(OH)2 process performance.
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