Calcination of Limestone

Several aspects of the calcination of Michigan limestone were investigated: the rate of calcination of limestone spheres with diameters from 1.8 to 14 cm at temperatures from 800° to 1200°C by a thermogravimetric method; the temperature profile across limestone during calcination; the excess pressure developed in the interior of the stone during calcination; the pore surface area, pore volume, and effective gas diffusivity in burnt lime; and the microstructure of polished and fracture surfaces. It is found that, over a wide range of experimental conditions, the difference between the surface and center temperatures remains essentially unchanged during the latter half of calcination. From the rate of weight loss after about 50% calcination, the value 0.00126 cal per cm sec degree is found for the effective thermal conductivity of burnt lime. From the excess internal pressure measurements, i.e., 0.4 to 6 cm Hg at 800° to 1200°C, the permeability of burnt lime is estimated to be within the range 8 X 10-' to 5 X 10' sq cm. With increasing temperature and time of calcination, the pore structure of burnt lime coarsens as indicated by a decrease in the pore surface area, pore volume, and an increase in the average pore size. For the temperature range 600° to 835°C the ratio (effective gas diffusivity/molecular diffusivity) is about 0.18.