THERMAL-ANALYSIS OF GROUND DOLOMITES

The characteristics of ground dolomites which have undergone a change in crystal structure and of those almost free from structural change, were studied mainly by thermal analysis. Firstly, pulverized samples passed through a 325-mesh Tyler sieve were ground further for 1, 2, 3, 5 and 24 h in order to observe their change with progressive grinding. The X-ray diffraction (XRD) and IR patterns and TG-DTA curves obtained in flowing 100% CO2 gas indicated a gradual loss of the cation ordering characteristic of dolomite structure up to a grinding period of 24 h when a new phase of lower ordering appeared; this confirmed the results of earlier investigators. To characterize further the gradually developing new phase which was not apparently distinguished by XRD or IR analysis, TG-DTA curves were obtained under varying CO2 partial pressures. It was found that the new phase decomposed in a straightforward manner, similarly to that of magnesite and quite differently from that dolomite, its decomposition temperature and rate were dependent upon the CO2 partial pressure. In addition, the influence of grain size and particle size distribution on the temperature and shape of the endothermic peak of the DTA curve was studied. It was found that the peak shape is affected by the grain size, and moreover that the temperature range of the endothermic peak is affected by the size and, more importantly, the size distribution of the particles.