Factors Affecting the Formation of Magnesite and Hydromagnesite from Direct Aqueous Carbonation of Thermally Activated Lizardite

Carbon capture utilisation and storage (CCUS) comprises a myriad of evolving technologies, with one of the most widely studied being direct aqueous mineral carbonation. This work discloses some of the factors which influence the formation of magnesium carbonate phases produced during direct aqueous carbonation; hydromagnesite and the targeted magnesite phase. A minimum stirring speed (100 rpm) was tested under standard carbonation conditions (130 bar, 150 C, 0.64 M NaHCO₃ and 15% solid ratio) for 1 hour. Aliquots of the reacting mixture were sampled regularly in order to gain insight into the reaction mechanism. At the minimum stirring speed studied, two Mg-carbonate phases were observed (hydromagnesite and magnesite) in all samples. By increasing the stirring speed to moderate and maximum impellor speed rates (450 and 600 rpm), hydromagnesite was observed only at the earlier stages of reaction; the first stage was during the stage when the reactor is heated to 150 áµ’C prior to the introduction of CO₂ into the system and the second stage denoted as time zero, when the desired pressure (130 bar) was reached by the booster. The position of the impeller was another factor which was examined. In these experiments, the top impeller was shifted just below the surface of the slurry in an effort to reduce the surface tension of the liquid and enhance the vorticity of the reacting solution. A dramatic change in the Mg carbonate phase produced was observed (at the minimum stirring speed) where hydromagnesite was observed after heating stage (in the absence of CO₂) and at time-zero where subsequently all hydromagnesite appears to be transformed to magnesite. In contrast, the influence of sodium bicarbonate was studied to examine its effect on the Mg carbonation yield and the phases thus formed. This series of carbonation experiments were performed with and without NaHCO_3. Carbonated samples were analysed by TGA-MS and XRD. In experiments where NaHCO₃ was absent, all the carbonated samples contain both hydromagnesite and magnesite whereas in reactions which were undertaken in the presence of NaHCO₃, hydromagnesite was observed only at the initial stages after heating stage (in the absence of CO₂) and at time-zero. By introducing NaHCO₃ into the reaction, it is speculated that carbonate ions are generated (CO₃^2-) and the ratio of Mg to carbonate ion will be close to or in excess of stoichiometric, thus favouring the formation of magnesite as per the chemical reaction shown below:

Mg²⁺_(aq) + CO₃^2- _(aq) → MgCO_3(s)

To favour the formation of hydromagnesite, less carbonate ions are needed and the stoichiometric ratio of Mg to carbonate ion is 0.8 as can be seen in the chemical reaction below:

5 Mg²⁺ _(aq) + 4CO₃^2- _(aq)+ 2OH^-_(aq)+ 4H₂O_(l) ↔ Mg₅(CO₃)₄(OH)₂.4H₂O_(s)