Particle size distribution and structural changes in limestone ground in planetary ball mill

Abstract

The dry grinding of limestone in a planetary ball mill was investigated as a function of the diameter of the grinding balls and the revolution speed of the mill. The tests were carried out with the same feed size (75×250μm) at variable times up to 240min. The analyses of the mean particle size, the specific surface area and the dispersion in size distribution showed that the grinding rate slowdown when ~50% of the particles became smaller than 20μm. Among the grinding conditions investigated here, that one set with 10mm balls, 200rpm and 60min showed to be suitable to achieve mean particle size smaller than 10μm. The scanning electron microscopy was useful to explain the decrease in the grinding rate by the effect of agglomeration of fines lying on partially broken particles. The agglomeration was attributed to the increase in particle surface energy that was promptly associated with the creation surface defects and bulk distortions detected by electron paramagnetic resonance (EPR) spectroscopy. The X-ray diffracting peaks of dolomite and other accessory minerals (talc, quartz and biotite) present in the limestone decreased with prolonged grinding. Differential thermal analysis revealed that the activation energy related to MgCO3 decomposition in dolomite decreased with the grinding time increasing and a simple relation was found between the energy deposited by the mechanical action and the specific surface area of ground particles.