A pinboard by
Sana Zahid

PhD Student, Murdoch University


The aim of my project is to provide the energy efficient solution for the large-scale implementation of mineral carbonation for the safe and permanent disposal of CO2 at the rate of gigatons/year. The abundance and well-distributed resources and natural weathering of serpentine minerals (hydrated magnesium silicates) to stable carbonates make it one of the most favourable raw material for CO2 sequestration at the global scale. However, slower dissolution kinetics are a major challenge for commercialization of mineral carbonation. Heat treatment results in the removal of structurally bound hydroxyl groups(dehydroxylation) which leads to increased reactivity through partial amorphisation of mineral. Hence, understanding the role of minerological changes during dehydroxylation is crucial for providing energy efficient solution. Moreover, the kinetic modelling using iso-conversional "Model-free" methodology is a key to estimate the accurate and reliable kinetic parameters. Hence, the implementation of isoconversional kinetic modelling will be helpful for the prediction and optimisation of mineral carbonation process. Moreover, the estimated activation energies will be helpful to optimise the reactor design.


Thermal activation of antigorite for mineralization of CO2.

Abstract: This contribution demonstrates the sensitivity of antigorite dehydroxylation to treatment conditions and discusses the implications of the observations for scientific (i.e., dehydroxylation kinetics) and technological (i.e., energy efficient conditions and design of practical activation reactors) applications. At present, the energy cost of dehydroxylation of serpentinite ores represent the most important impediment for a large scale implementation of sequestering CO(2) by mineralization. We have analyzed changes in antigorite's derivative thermogravimetric curves (DTG) and deduced factors affecting the mass loss profiles. The imposed heating rate, type of purge gas, type of comminution and sample mass all influence the dehydroxylation curve. However, the results show no influence of material of construction of the heating vessel and flow rate of the purge gas. We report an important effect of oxidation of Fe(2+) under air purge gas that occurs prior to dehydroxylation and leads to formation of hematite skins on serpentinite particles, slowing down subsequent mass transfer and increasing the treatment temperature. From the process perspective, 75 μm particles afford optimal conditions of temperature and rate of dehydroxylation. Overall, the practical considerations, in thermally activating serpentinite ores for storing CO(2) by carbonation, comprise rapid heating, proper size reduction, prior demagnetisation, and fluidization of the powder bed.

Pub.: 12 Dec '12, Pinned: 20 Oct '17

In situ high-temperature X-ray diffraction and spectroscopic study of fibroferrite, FeOH(SO4)·5H2O

Abstract: The thermal dehydration process of fibroferrite, FeOH(SO4)·5H2O, a secondary iron-bearing hydrous sulfate, was investigated by in situ high-temperature synchrotron X-ray powder diffraction (HT-XRPD), in situ high-temperature Fourier transform infrared spectroscopy (HT-FTIR) and thermal analysis (TGA-DTA) combined with evolved gas mass spectrometry. The data analysis allowed the determination of the stability fields and the reaction paths for this mineral as well as characterization of its high-temperature products. Five main endothermic peaks are observed in the DTA curve collected from room T up to 800 °C. Mass spectrometry of gases evolved during thermogravimetric analysis confirms that the first four mass loss steps are due to water emission, while the fifth is due to a dehydroxylation process; the final step is due to the decomposition of the remaining sulfate ion. The temperature behavior of the different phases occurring during the heating process was analyzed, and the induced structural changes are discussed. In particular, the crystal structure of a new phase, FeOH(SO4)·4H2O, appearing at about 80 °C due to release of one interstitial H2O molecule, was solved by ab initio real-space and reciprocal-space methods. This study contributes to further understanding of the dehydration mechanism and thermal stability of secondary sulfate minerals.

Pub.: 27 May '16, Pinned: 20 Oct '17

High-pressure behaviour of serpentine minerals: a Raman spectroscopic study

Abstract: Four main serpentine varieties can be distinguished on the basis of their microstructures, i.e. lizardite, antigorite, chrysotile and polygonal serpentine. Among these, antigorite is the variety stable under high pressure. In order to understand the structural response of these varieties to pressure, we studied well-characterized serpentine samples by in situ Raman spectroscopy up to 10 GPa, in a diamond-anvil cell. All serpentine varieties can be metastably compressed up to 10 GPa at room temperature without the occurrence of phase transition or amorphization. All spectroscopic pressure-induced changes are fully reversible upon decompression. The vibrational frequencies of antigorite have a slightly larger pressure dependence than those of the other varieties. The O–H-stretching modes of the four varieties have a positive pressure dependence, which indicates that there is no enhancement of hydrogen bonding in serpentine minerals at high pressure. Serpentine minerals display two types of hydroxyl groups in the structure: inner OH groups lie at the centre of each six-fold ring while outer OH groups are considered to link the octahedral sheet of a given 1:1 layer to the tetrahedral sheet of the adjacent 1:1 layer. On the basis of the contrasting behaviour of the Raman bands as a function of pressure, we propose a new assignment of the OH-stretching bands. The strongly pressure-dependent modes are assigned to the vibrations of the outer hydroxyl groups, the less pressure-sensitive peaks to the inner ones.

Pub.: 01 Jun '04, Pinned: 20 Oct '17

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

Abstract: The dehydroxylation reactions of chrysotile Mg3Si2O5(OH)4 and brucite Mg(OH)2 were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO2 in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (‘time to a given fraction’ and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress-resolved (‘α’-resolved) course of the apparent activation energy Ea and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing Ea (α ≤ 15 %, 240–300 kJ/mol), before coming down and forming a plateau (15 % ≤ α ≤ 60 %, 300–260 kJ/mol). The reaction ends with an increasing Ea (α ≥ 60 %, 260–290 kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in Ea versus α (160–110 kJ/mol). In contrast to that, the decomposition of brucite under CO2 delivers a dynamic course with a much higher apparent Ea characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent Ea comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in Ea versus α plots.

Pub.: 05 Nov '13, Pinned: 20 Oct '17

A dehydroxylation kinetics study of brucite Mg(OH) 2 at elevated pressure and temperature

Abstract: Abstract We performed an in situ dehydroxylation kinetics study of brucite under water-saturated conditions in the pressure and temperature ranges of 593–633 K and 668–1655 MPa using a hydrothermal diamond anvil cell. The kinetic analysis of the isothermal–isobaric data using an Avrami-type model involving nucleation and growth processes yields values for the dehydroxylation rate and reaction order compatible with a reaction mechanism limited by the monodimensional diffusion of water molecules from structural OH groups. Our results show a negative pressure dependence on the reaction rate k and a positive temperature dependence on the k. The dehydroxylation of brucite yields an activation volume ∆V value of 5.03 cm3/mol. Following the Arrhenius relationship, the apparent activation energy E a of the process is calculated to be 146 kJ/mol within the experimental P–T ranges. It is determined that the fluid production rates varying from 4.4 × 10−7 to 10.7 × 10−7 \({\text{m}}_{\text{fluid}}^{3} \;{\text{m}}_{\text{rock}}^{ - 3} \;{\text{s}}^{ - 1}\) are a few orders of magnitude greater than the strain rate of the mantle serpentinites, which may be fast enough to result in the brittle fracture of rocks. Moreover, the rate of fluid production will be enhanced when brucite occurs in the non-/low H2O environments of the subduction zone.

Pub.: 07 Nov '16, Pinned: 20 Oct '17