Crystallization and fracture: formation of product layers in sulfation of calcined limestone

Sulfation experiments were carried out and product layers were characterize d to investigate the reaction kinetics and to test a Crystallization and Fr acture (CF) model describing the: mechanism of formation of product layers. Sintered samples of calcined Limestone were sulfated in a quartz reactor f or short and extended times in order to obtain initial and fully developed product layers. The product layers were characterized by means of Scanning Electron Microscopy (SEM) visualization and measurements of BET surface are a and pore size distribution. At lower temperatures, the sulfation reaction ceased after a relatively shea time, whereas at higher temperatures, the r eaction continued at an approximately constant rate for as long as 30 h. Th e reaction product was formed by crystallization. The product "layer" forme d in the early stages of the reaction was not a true layer, but isolated nu clei and crystals. The "continuous" product lever formed in the later stage s of the reaction was found to be a monolayer of individual crystals with p ores of size 20-30 Angstrom along the boundaries. The product layer was mor e porous when developed from larger stable nuclei formed during the initial reaction at higher temperatures and lower SO2 concentrations. These observ ations support the mechanism described by the CF model. (C) 2000 Esevier Sc ience S.A. All rights reserved.