Reactivity of lime and related oxides. VII crystal size variations in calcium oxide produced from limestone

Abstract

AbstractThe calcination of limestone has been studied by determining the changes in the specific surface and crystallite size of the calcium oxide produced when limestone spheres are calcined for various times at each of a number of fixed temperatures either in vacuo or in air.At lower temperatures (below about 900°) as in rotary‐kiln lime‐burning, the crystallite size of the calcium oxide is determined more by the mode of recrystallisation of the oxide from its CaCO3‐like transition structure than by the sintering of the recrystallised oxide. Since the decomposition rate of the limestone progressively diminishes, the rate of nucleation in the recrystallisation of the oxide becomes lower, so that the crystallite size of the oxide formed in the outer sections of the spheres tends to be less than that formed in the inner sections. The differences in size are minimised if the evolved carbon dioxide is not quickly removed, for it enhances sintering of the calcium oxide.At higher temperatures (above about 900°), as in conventional lime‐kilning, the crystallite size of the calcium oxide depends mainly on the rate of sintering. Rapid decomposition of the limestone at these temperatures therefore produces lime more homogeneous in surface activity even after only short calcinations, since all of the lime crystallites in each sample still have approximately equal times in which to sinter; the crystallite size of the lime then becomes practically independent of that of the original calcium carbonate.