Co-utilisation of CO₂ and calcium silicate-rich slags for precipitated calcium carbonate production (part II)

Production of commercial calcium carbonate from calcium silicate-rich slag is an attractive concept for reducing CO₂ emissions. In order for this calcium carbonate to be marketable it has to be very pure, requiring separation of calcium from the slag prior to carbonation. In this study, a process concept that produces relatively pure calcium carbonate from slag is presented. It uses an aqueous solution of acetic acid to dissolve blast furnace slag, followed by removal of residues and evaporation of the acid to produce acetate, which is then dissolved in water before CO₂ is introduced. The alkalinity of the solution is raised by addition of sodium hydroxide, which causes calcium to precipitate as calcium carbonate. The fate of other elements released from the slag during the process was studied as well. It was found that while Si, Ti and Zr can be separated by facilitating gel formation, Al, V, Mn, Fe, and Ba require additional separation methods to produce pure calcium carbonate. The process also requires additional measures for preventing sulphuric vapour emissions. The economical feasibility of the proposed process was also investigated. The evaporation step was found to be expensive, unless the process could be integrated with a waste heat source. Recovery of the sodium acetate solution, a by-product of the carbonation step, was also considered necessary for the process to be economically feasible.