ABSTRACT

The formation of powdered calcite from slurries containing a calcium source and carbon dioxide (industrial carbonation route) is a complex process of considerable importance nowadays. In the absence of additives, the rhombohedral morphology can be obtained in precipitation processes by using solution routes but rarely by the industrial method, where the most common morphology of precipitated calcite is the scalenohedral one. Rhombohedral calcite with a very low degree of agglomeration, high specific surface area, and sizes going from few microns to nanocrystals can be obtained under supercritical conditions. The use of CO2 in a compressed

form facilitates the reduction of the reactor size to keep the desired production rate. Moreover, the use of ultrasounds coupled to scCO2treatment largely improves carbonation process kinetics. The scCO2carbonation process can be also applied to the in situ precipitation of calcite inside of the pores of cellulose paper or Portland cement, thus increasing the density and reducing water permeability and the pH of the material, which enhances the durability in certain applications. Cement carbonation not only generates a high added value product but also could help in CO2 capture and storage and, therefore, it is regarded as a sustainable process. Calcium-based CO2 solid sorbents for CO2 capture reveal a different efficiency of the carbonation/calcination cycle according to their origin, natural or synthetic, and the synthesis method. Under realistic conditions (decarbonation at 1173 K) and after 25 cycles of CO2 adsorption/desorption, scCO2-precipitated CaCO3 sorbents have a residual conversion value two times higher than that of natural limestone. All these different aspects of supercritical carbonation will be addressed in this chapter. 11.1 CO2 Carbonation ReactionNatural carbonation of portlandite mineral (calcium hydroxide, Ca(OH)2) is a well-known phenomenon associated with the weathering of alkaline rocks, which plays important roles in day-to-day life, such as the control of the fraction of carbon dioxide (CO2) in the atmosphere or the duration of stainless steel concrete structures placed in humid environments [1, 2]. Besides, the formation of calcium carbonate (CaCO3) by a reaction between Ca(OH)2 and CO2 is a complex process of considerable importance in the industrial, ecological, geochemical, and biological areas. The main CaCO3 polymorphs are calcite, aragonite, and vaterite [3]. Calcite is the thermodynamically stable phase at room temperature and atmospheric pressure. Aragonite is the high-temperature phase displaying a needle-like morphology and vaterite is the low-temperature phase, often found as spherical aggregates. Calcite has a principal rhombohedral crystal structure, bounded by the (104) face as the most stable surface. Besides a rhombohedral structure, a myriad of morphological variants is possible, underlining the

importance of the elongated scalenohedral calcite form, bounded by the (21-1) face. The carbonation reaction consists in bubbling CO2 gas through a concentrated aqueous slurry of Ca(OH)2 (Eq. 11.1). Ca(OH)2(s, H2O) + CO2(g) ↔ CaCO3(s) + H2O(l) (11.1) The solid-liquid-gas atmospheric carbonation of Ca(OH)2 is a slow process with low carbonation efficiency, mainly due to the low solubility of CO2 in water. In this respect, accelerated carbonation methods using compressed or supercritical carbon dioxide (scCO2) as a reactant have been described as efficient alternatives to the atmospheric process [4-9]. Formation of CaCO3 involves four main steps: dissolution of Ca(OH)2, formation of carbonate ions, chemical reaction, and crystal growth. The global process is schematized in Fig. 11.1a. Calcite formation has been described as proceeding through two consecutive fast and slow reaction stages (Fig. 11.1b) [10]. In the first fast stage, the heterogeneous precipitation of CaCO3on the Ca(OH)2 surface occurs. As the reaction proceeds and the conversion to carbonate increases, a layer of CaCO3 develops on the surface of the Ca(OH)2 particles, forming a core-shell structure [11]. The dissolution of Ca(OH)2 occurs in two stages [12]: First, the Ca(OH)2 particles chemically dissolve on the surface (Eq. 11.2, where Kps(2) is the solubility product), and second, Ca2+ ions diffuse away from the surface (Eq. 11.3). In the slow stage the reaction rate decreases, since the precipitation of CaCO3 inhibited Ca(OH)2dissolution and Ca2+s diffusion to some extent. Ca(OH)2 ↔ Ca2+s + 2OH-(solid surface) Kps(2)_298K = 10-5.3 (11.2) Ca2+s + 2OH-(solid surface) ↔ Ca2+b + 2OH-(bulk solution) (11.3) For the formation of carbonate ions, first the reagent CO2needs to enter the water phase and, therefore, solubility and mass transport resistance would be important parameters controlling the reaction rate. Figure 11.1c shows, in the pressure-temperature CO2phase diagram, different solubility values of CO2 in H2O at several standard working parameters at the vapor, liquid, and supercritical conditions. The resistance applied by the water medium to CO2

penetration can be stated in terms of density and surface tension. At high pressures, the densities of the water and CO2 phases are similar and less critical shaking intensity is required to overcome the interfacial tension (Fig. 11.1d).