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Thermal dehydroxylation kinetics and investigation of factors affecting the dehydroxylation of serpentine minerals to improve CO2 sequestration through mineral carbonation

Zahid, Sana (2020) Thermal dehydroxylation kinetics and investigation of factors affecting the dehydroxylation of serpentine minerals to improve CO2 sequestration through mineral carbonation. PhD thesis, Murdoch University.

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Thermal dehydroxylation of serpentine minerals (Mg3Si2O5(OH)4) is a crucial step for mineral carbonation, a promising technique for the safe and permanent disposal of CO2 emissions. Thermal dehydroxylation enhances the dissolution kinetics of these minerals through amorphisation of mineral structure but at the expense of an additional energy penalty. Therefore, this thesis aims to generate an in-depth understanding of structural changes and dehydroxylation kinetics under different heat-treatment conditions to provide an energy-efficient solution for the commercialisation of mineral carbonation.

Isoconversional kinetic analysis based on thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses demonstrate the multistep nature of antigorite dehydroxylation, with activation energies (Eα) varying between 290 and 515 kJ mol-1. The high resolution, in-situ synchrotron powder X-ray diffraction (PXRD) for the first time enables the identification of two amorphous metaserpentine components (α and β) along with minor semi-crystalline chlorite-like γ-metaserpentine formation during antigorite dehydroxylation. The amorphous α and β-metaserpentine components originate simultaneously and then transform to forsterite and enstatite, respectively, and chlorite-like formation reveals an additional reaction pathway for the Al2O3-rich antigorite dehydroxylation process. The combination of TGA-DSC-based kinetics and PXRD data illustrates that only ~49% dehydroxylation of Al-rich antigorite can be achieved before forming less-reactive forsterite and Eα increase.

The effect of the partial pressure of water vapours (PH2O), grain size, and mineralogy on the structural changes during dehydroxylation of serpentine minerals by in-situ synchrotron PXRD illustrates that the lizardite is a more suitable feedstock for mineral carbonation compared to antigorite because it: a) produces almost all amorphous content before forsterite formation; b) yields 2-3 times higher amorphous content than antigorite; and, c) reduces the energy requirements to nearly half due to high amorphous production and a lower dehydroxylation temperature. In particular, low PH2O and small-sized lizardite particles make this mineral especially suitable for mineral carbonation. The small-sized particles also favour antigorite dehydroxylation, and the high PH2O does not significantly affect its dehydroxylation. Overall, higher dehydroxylation temperatures and overlapping of amorphisation with forsterite formation make antigorite infeasible for subsequent mineral carbonation. Moreover, this study also suggests that the amorphous α and β-metaserpentine components are precursors of forsterite, and enstatite, respectively, in both serpentine polymorphs, i.e., antigorite and lizardite. In antigorite, the formation of both amorphous components coincides and reaches the maximum amorphous content at the same temperature. In contrast, in lizardite, the β-metaserpentine lags the α-metaserpentine component, and it is nearly 80% at the higher amorphous production. This coexistence of both amorphous components also exhibits the maximum possible exploitation of serpentine feedstock for mineral carbonation due to the infeasibility of the preferential production of one amorphous component over the other.

Also, in-situ synchrotron-PXRD yielded measurements for the isoconversional kinetic analysis. As opposed to TGA-DSC, which provides insight into the coupled dehydroxylation-amorphisation-crystallisation process, synchrotron-PXRD can resolve processes affecting individual mineral phases. This enables kinetic analysis of antigorite dehydroxylation separately from forsterite formation, demonstrating the initial gradual increase in Eα is due to diffusion limitation, whereas the sharp rise in Eα during the later stages of dehydroxylation occurs because of forsterite formation. Consequently, this study highlights that the trade-off between amorphicity and forsterite formation is vital for developing an economical heat treatment of antigorite to store CO2 by mineral carbonation.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Chemistry and Physics
Supervisor(s): Oskierski, Hans, Dlugogorski, Bogdan, Senanayake, Gamini, Altarawneh, Mohammednoor and Brand, H.
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