CRYSTAL GROWTH & DESIGN
Oriented Nucleation and Growth of Anhydrite during Direct Sulfation of Limestone
2008 VOL. 8, NO. 4 1181–1185
Guilin Hu,* Kim Dam-Johansen, and Stig Wedel Department of Chemical Engineering, Technical UniVersity of Denmark, 2800 Lyngby, Denmark ReceiVed May 14, 2007; ReVised Manuscript ReceiVed December 7, 2007
ABSTRACT: The direct sulfation of limestone (Iceland Spar) was studied at 973 K in a fixed-bed reactor. Scanning electron microscopy examinations of the sulfated limestone particles show that the sulfation process involves oriented nucleation and growth of the solid product, anhydrite. The reason for the occurrence of oriented nucleation and growth of the solid product is related to a close two-dimensional lattice match between the lattice plane {1,0,4} of calcite and the lattice plane {1j,1,0} of anhydrite.
1. Introduction The direct sulfation of limestone is defined as the sulfation reaction between SO2 and limestone in the uncalcined state and can be expressed by the following overall reaction: CaCO3(s) + SO2(g) + 0.5O2(g) f CaSO4(s) + CO2(g) (1) This reaction is important for flue gas desulfurization at high temperatures such as during pressurized fluid bed combustion (PFBC) and cement production.1–4 In the past few decades, this reaction has been studied extensively with a focus on the apparent reaction kinetics. However, little is known about the crystallographic aspects of this reaction. Formation of a new solid phase during a reaction such as a gas–solid or a solid–solid reaction at high temperatures usually involves nucleation of the new phase and subsequent growth.5 The nucleation and growth can be oriented when the lattice structures of the substrate (the solid reactant) and the solid product have a two-dimensional (2D) similarity (epitaxy) or a three-dimensional (3D) similarity (topotaxy). This paper presents and discusses crystallographic phenomena observed in the study of the direct sulfation of Iceland Spar, which is part of a project work for SO2 emission control during cement production.
Figure 1. Illustration of the reactor setup. Table 1. Chemical Composition of Iceland Spara CaCO3 content: (w %) elemental analysis (w %): Mg S Ca Mn Fe Sr other elements a
>99.5 0.005 0.04 40 0.006 0.02 0.02
