One of the major problems facing mankind in 21st century is global warming which is thought to beinduced by increasing concentrations of CO2 in the atmosphere. Carbon dioxide sequestration by means of minerals is arguably one the most promising routes for carbon storage as there are large deposits of silicate minerals worldwide. Mineral carbonation results in the storage of CO2 in solid form as a stable, environmentally benign mineral carbonate. There are two main types of processes for mineral carbonation: gas-solid mineral carbonation and aqueous mineral carbonation. Although different processes have been suggested for mineral CO2 sequestration during the last two decade, none of the pathways have yet demonstrated that it can be the basis of a commercial process. Furthermore, the experimental data on gas-solid carbonation of serpentine is quite limited. It is important therefore to develop a process having an efficient, economical and low waste residue for the carbonation of minerals.
In this research both routes were examined. Initially naturally occurring serpentine was characterized by evaluating its porosity, density, crystalline structure, surface topography and identifying the key elements. Direct and two-step gas-solid carbonation of serpentine were investigated. In the first step, dehydroxylation of serpentine as a pre-treatment option was considered for natural serpentine to increase its reactivity. The dehydroxylation kinetics were studied by non-isothermal thermogravimetry (TGA). The results indicated that the thermal decomposition of serpentine proceeds via the removal of physisorbed water, and subsequently of the hydroxyl group. Kinetic modelling of the dehydroxylation shows that the reaction follows a three dimensional diffusion-controlled mechanism in the particles. The effect of the heating rate and particle size on the dehydroxylation reaction has been investigated, and the results were found to be consistent with diffusion controlled kinetics. The gas-solid heat transfer resistance was shown to influence the results at high heating rate and large particle size. Carbonation experiments of produced dehydroxylated serpentine and original serpentine were performed using High Pressure Volumetric Analyser (HPVA) at high pressure and high temperature of 482˚C and 40 bar, respectively. The results identified that conversions were insignificant. Therefore the reaction was extremely slow to be industrially viable and feasible.
The best approach through aqueous route was a multi-stage mineral carbonation where the first stage was considered for Mg dissolution of serpentine at high temperature and low pH and the second stage was for Mg carbonation at high pH. Dissolution of Mg from serpentine is favourable with temperatures at the boiling point of the solution and 3 hours of residence time. CO2 equilibria in the solution were modelled in MATLAB and were shown that pH was the dominant factor in
precipitating carbonates. It was also revealed that the effect of pH was amplified by mildly increasing the partial pressure of CO2. The role of pH by a novel concept which was to use weakly basic tertiary amines for binding with H+ ions to enable the high pH needed for carbonation stage. The pH needed for magnesium carbonate precipitation was found to be approximately 7.8 to 8.2. Both triethylamine and tripropylamine were able to raise the pH to this level at 18°C using residence times of approximately 1 hour. The acid dissociation constant for protonated tertiary amines increases with increasing temperature and so the approach is to heat the rich amine (loaded with acid) to strip the acid off and therefore provide the high temperature and low pH needed for serpentine dissolution in first stage. The amine can be thought of as a regenerable buffer by means of applying heat. The pH swing for the tertiary amines was found to be approximately 2.5 pH units between 5 and 85°C, suggesting that an amine capable of achieving a pH of 8.2 at low temperature generates a pH of 5.7 in solution when heated to 85°C.
The overall conclusions of this body of research are that: Dehydroxylation process of serpentine was not an effective pretreatment option for gas-solid mineral carbonation; gas-solid mineralization of CO2 was found to be favoured at higher pressures above 40 bar; the key factor that influences carbonation reaction is the pH of a solution; use of tertiary amines for pH-swing process of mineral carbonation is technically feasible in that a conception of regeneration after carbonation was
presented. The concept of using regenerable buffer can be used in a variety of different ways and the knowledge could be transferred to develop other processes that need pH-swing. This approach could assist to design a novel process for an aqueous mineral carbonation in order to enable permanent storage of CO2.