Sintering of High-calcium Quicklime

The processes described in section 15.3.2 also apply to limestones containing significant quantities of magnesium carbonate. From a practical viewpoint, the calcination of such limestones does not differ significantly from that of high-cal-cium limestones. 

When a dense calcitic limestone is calcined at temperatures close to 900 °C, the volume of individual particles increases very slightly until about 60 % of the limestone has dissociated, and then decreases slightly to about 98 % of the original volume at the point when calcination is complete [15.8]. 

Calcination at very high temperatures has been shown to cause significant sintering of the surface layer of lime, while dissociation is still in progress under extreme conditions this can interfere significantly with the dissociation process [15.1]. 

The sintering process after the eompletion of dissoeiation is of more relevance to practical lime-buming, because of its effect on the reactivity of the lime. Fig. 15.6 shows the effect of temperature and time on sintering, as measured by the variation of apparent density, of a quicklime produced from a dense, high-cal-cium limestone [15.8]. 

Scanning electron micrographs of quicklime demonstrate the dramatic change in structure resulting from the sintering process, for the same high-calcium quicklime (Fig. 15.7). The effects of sintering on lime reactivity are described in section 13.2. 

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