Carbonation Mechanisms and Modelling

Abstract

From the view point of reaction kinetics, many researchers have attributed the rate-determined step of aqueous carbonation to the reacted (product) layer diffusion. Therefore, this suggests that a well-designed reactor to enhance the mass transfer between the gas, liquid, and solid phases is needed to facilitate the car-bonation reaction and increase the carbonation conversion. This chapter provides the principles and mechanisms of carbonation reaction using alkaline solid wastes. The kinetics of the major three steps of carbonation, i.e., metal ion leaching, CO 2 dissolution, and carbonate precipitation, are illustrated. Moreover, several classical heterogeneous kinetic models, e.g., shrinking core model and surface coverage model, are presented. Furthermore, the modeling of mass transfer for carbonation in various reactors and/or processes is summarized and discussed. 7.1 Carbonation Mechanisms 7.1.1 Principles As discussed in Chap. 5, the process chemistry of accelerated carbonation using alkaline solid wastes can be briefly divided into three steps: • Step 1: Contemporary dissolution of CO 2 into a liquid phase and conversion of carbonic acid to carbonate/bicarbonate ions • Step 2: Dissolution of CO 2-reactive species from a solid matrix (irreversible hydration) • Step 3: Consequent nucleation and precipitation of carbonates At first, the leaching of Ca-bearing compounds in alkaline solid waste would directly generate the Ca 2+ in the solution. Secondly, gaseous CO 2 can rapidly dissolve into the alkaline solution, where the predominant carbonate ions (CO 3 2−) could reduce the pH of the solution. Since the CO 2 continuously dissolved into the solution during the carbonation, the pH value would decrease gradually to 6.3,