Analysis of Ash-Forming Mineral Matter in Raw and Supercleaned Coals by Automated Image Analysis-Scanning Electron Microscopy

Abstract

Automated image analysis (AIA) was used in conjunction with scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) spectrometry to characterize directly the ash-forming mineral matter in raw and supercleaned samples of Illinois No. 6 and Pittsburgh No. 8 coals. The ground coals (70-80% less than 200 mesh) were cleaned to about 3% ash content by float-sink separation at 1.3 specific gravity. Samples of the raw and float material were analyzed for mineral matter phase and size distribution by the AIA-SEM technique. For the Illinois No. 6 coal, more than 90% of the mineral matter consisted of pyrite, kaolinite, illite or quartz, which were more or less uniformly distributed among the various particle sizes. The effectiveness of cleaning was gradually increasing with increase in mineral particle size. Removals of mineral matter ranged from 75% for the smallest particles (less than 4 µm in diameter) to 100% for particles larger than 36 µm in diameter. The Pittsburgh coal was significantly different in both the character of the raw sample and the extent of cleaning observed. Compared to the Illinois No. 6 coal, the pyrite in this sample was relatively more coarse, while the other minerals were more fine-sized. The cleaning process removed more of the large-size material, while the finely grained material was relatively untouched.