Aqueous carbonation of the potassium-depleted residue from potassium feldspar–CaCl₂ calcination for CO₂ fixation

Based on the academic thought of carbon capture and utilization, a novel process to integrate the potassium extraction from the insoluble potassium feldspar, industrial waste utilization, and the subsequent CO₂ fixation using the resultant potassium-depleted residue was proposed in our previous studies. The potassium-depleted residue comprises several Ca-bearing phases, namely wollastonite (CaSiO₃), pseudowollastonite (Ca₃Si₃O₉), Cl-mayenite (Ca₁₂Al₁₄O₃₂Cl₂), and anorthite (CaAl₂Si₂O₈), which are potential materials for fixation of CO₂ via carbonation. In this study, carbonation of the residue was examined with focuses on the effects of reaction temperature, initial CO₂ pressure, particle size of the residue, and reaction duration on the carbonation of these Ca-bearing phases. The results demonstrated that both the temperature and CO₂ pressure significantly affect the carbonation, while the residue particle size has only minor influence. At 1 MPa CO₂ pressure, the carbonation of these components was dominant at different reaction temperatures. Almost complete carbonation of the pseudowollastonite could be achieved at 75 °C, while significant carbonation of the wollastonite takes place above 100 °C. However, the Cl-mayenite and anorthite are incapable of carbonation even at 200 °C. Increasing the CO₂ pressure to 4 MPa can lead to a distinct carbonation of the Cl-mayenite at 150 °C but the anorthite remains untouched. At 1.5 MPa CO₂ pressure and 150 °C, with the increasing reaction time, the following Ca-bearing species were successively carbonated: first the pseudowollastonite in 5 min after the reaction started, the wollastonite in 5–15 min, and then simultaneously the wollastonite and the pseudowollastonite in 15–45 min, while the carbonation of Cl-mayenite do not begin even after 120 min. A priority sequence of carbonation of these Ca-bearing minerals was determined as follows: pseudowollastonite > wollastonite > Cl-mayenite > anorthite. The trend is in agreement with the results of thermodynamic calculation. Compared to the carbonation of natural wollastonite, the synthesized wollastonite contained in the potassium-depleted residue seems to be more active in carbonation.