Alkali-Based Extraction of Mg(OH)2 from Mg Silicate Minerals Via Solid State Reaction with NaOH

Alkali-based extraction of Mg(OH)₂
from Mg silicate minerals  via solid state reaction with NaOH

S. Madeddu¹,
M. Priestnall², E. Godoy³, V. Kumar⁴
and H. Kinoshita¹

¹ Department
of Materials Science and Engineering, The University of Sheffield, Sheffield,
United Kingdom.

² Cambridge Carbon
Capture Ltd., Cambridge, United Kingdom.

³ Polarcus DMCC,
Dubai, U.A.E.

⁴ Department of Materials Science and
Metallurgy, University of Cambridge, Cambridge, United Kingdom

*Corresponding
author: mtq11sm@sheffield.ac.uk.

Mg
bearing materials can be used to sequester CO₂ via carbonation
reaction by forming stable Mg carbonate minerals which can be safely disposed
and assure the long-term storage of CO₂^1,2. Ultramafic
rocks such as dunite and serpentinite³ provide large potential for
CO₂ sequestrationas they contain high wt% of Mg and are
widely available in natural deposits¹. The main components of dunite
and serpentinite are forsterite (Mg₂SiO₄) and the
polymorphs of the serpentine group, i. e. antigorite, lizardite and chrysotile
(Mg₃Si₂O₅(OH)₄)⁴.
Although the carbonation of forsterite and serpentine is energetically
favoured, the kinetic of reaction is too slow for large scale applications^1,2.

To accelerate the kinetic of carbonation, the Mg silicate
minerals can be chemically pre-treated to extract Mg^1,2. The Mg can be extracted as Mg(OH)₂^5,6,
with several advantages: (1) Mg(OH)₂ is an ideal material for
mineral carbonation because it is alkaline and has a faster kinetic of
carbonation reaction compared to the original Mg silicate minerals⁷, (2) Mg(OH)₂ is a rare compound
in nature whereas Mg silicate minerals are widely diffuse and offer large
capacity for Mg(OH)₂ production^1,2, (3) Mg(OH)₂
can be potentially used to capture CO₂ directly from flue gases and
avoid the expensive pre-separation of CO₂⁸, (4) Mg(OH)₂ can be selectively
moved from the mine site to the CO₂ capture/storage location
reducing the amount of feedstock materials transported.

The technologies developed
for the production of Mg(OH)₂ from Mg silicate minerals involve the
reaction with acidic solutions or salts^5,6. These processes require
a second stage to increase the pH of the system and allow the formation of
Mg(OH)₂^5,6. The shift of pH requires the consumption of
either additional alkaline reactants⁵,or recovered
alkaline by-products emitted during the process⁶, which increases
the cost and energy demand of these technologies⁹. The pH shift can
be avoided by the direct formation of Mg(OH)₂ with the alkali-based
chemical pre-treatment of Mg silicate minerals.

Previously, we investigated the alkali-based conversion of
dunite, composed by ~ 73 wt% forsterite, into Mg(OH)₂ using highly
concentrated NaOH (aq) systems (Eq. (3))¹⁰.
 

(3)

It was demonstrated that the mineral components of dunite can
be mostly substituted with Mg(OH)₂ in highly concentrated NaOH (aq)
systems. It was also identified that the main point for improvement was the
large amount of NaOH required to achieve significant Mg(OH)₂ formation¹⁰.

In the present study, a new solid-state reaction was
investigated in the attempt to reduce the NaOH consumed. The powdered dunite
was mixed with reagent grade NaOH with different mass
ratios and the addition of small amounts of H₂O was tested for
selected samples. The samples were heated in an electric furnace at different
temperatures and durations to study the kinetic of reaction as well as the
effect of NaOH and H₂O content at different temperatures and times.
The reaction products were washed with distilled water to separate the soluble
phases from the solid products which were analysed via X-ray Diffraction (XRD),
Thermogravimetry (TG) and Scanning Electron Microscopy (SEM). The concentration
of Mg(OH)₂ in the reaction products was estimated via TG and
Rietveld Refinement Quantitative Phase Analysis (QPA).

The results showed that the Mg contained
in dunite can be converted into Mg(OH)₂ via solid state reaction
with NaOH at relatively low temperature, i. e. < 200 °C. The NaOH consumed
was reduced by > 93 % compared to the
aqueous reaction previously reported¹⁰, achieving ~ 57 wt% Mg(OH)₂ in the reaction products. The
consumption of H₂O was also decreased by >
98 % or avoided completely. The mechanism of reaction
was studied and the kinetics of Mg(OH)₂ extraction will be
discussed. The obtained results suggest that the amount of NaOH required
for the extraction of Mg(OH)₂ from dunite mineral components can be
drastically reduced via solid state reaction maintaining significant Mg(OH)₂
formation.  

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