Principles of Accelerated Carbonation Reaction
Industrial alkaline solid wastes are ideal accelerated carbonation materials due to their availability and low cost. These materials are generally rich in calcium content and often associated with CO2 point source emissions so no mining is needed and the
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Principles of Accelerated Carbonation Reaction
Abstract Industrial alkaline solid wastes are ideal accelerated carbonation materials due to their availability and low cost. These materials are generally rich in calcium content and often associated with CO2 point source emissions so no mining is needed and the consumption of raw materials is avoidable. This chapter provides the principles and definitions of accelerated carbonation reaction using alkaline solid wastes. Two types of carbonation processes, i.e., direct and indirect carbonation, are briefly discussed from the theoretical considerations, including process chemistry and key performance indicators. The performance and application of both direct and indirect carbonation processes can be found in detail in Chap. 8.
5.1 5.1.1
Principles and Definitions Theoretical Considerations
Accelerated carbonation was first proposed by Seifritz [1], which involved alkaline materials reacting with high-purity CO2. The concept behind accelerated carbonation processes is to mimic natural weathering, where gaseous CO2 reacts with metal-oxide-bearing materials in the presence of moisture. In this case, the reaction can be accelerated to a timescale of a few minutes or hours. After accelerated carbonation, stable and insoluble carbonates will be formed, where CaO and MgO are the most favorable metal oxides in reacting with CO2 [2]. In principle, the affinity of oxides for carbonate formation depends on the following: • • • •
chemisorption strength to CO2 gas (especially for gas–solid interface); number of basic sites at the surface (especially for gas–solid interface); solubility product constant, i.e., Ksp (in aqueous carbonation reaction); and total content in solid particle.
© Springer Nature Singapore Pte Ltd. 2017 P.-C. Chiang and S.-Y. Pan, Carbon Dioxide Mineralization and Utilization, DOI 10.1007/978-981-10-3268-4_5
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5 Principles of Accelerated Carbonation Reaction
Therefore, the affinity of oxides varies as follows: basic oxides (CaO, MgO) > amphoteric oxides (Al2O3, Cr2O3, TiO2, MnO, iron oxides) > acidic oxides (SiO2). From a thermodynamic point of view, both alkaline earth metals (e.g., Ca and Mg) and alkali metals (e.g., Na and K) can be carbonated [3, 4]. However, alkali carbonates and/or bicarbonates are soluble in water, which will result in releasing CO2 back into the atmosphere and therefore considered to be unsuitable for the long-term storage of CO2. Consequently, the capacity of CO2 fixation by these alkaline residues depends directly on the proportion of binary oxide (CaO and MgO) and/or hydroxide (Ca(OH)2 and Mg(OH)2) content in the matrix. Moreover, CaO offers more potential for chemisorptions of CO2 than MgO due to its basic characteristics [5]. Furthermore, a number of other metals, such as Mn, Fe, Co, Ni, Cu, and Zn, are impractical for carbonation due to their unique and precious features for recovery and utilization. Accelerated carbonation can be classified into two main types: mineral carbonation (as discussed in this chapter) and alkali
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