QUANTITATIVE MINERAL DETERMINATIONS OF INDUSTRIAL COAL ASH

The majority of coal ash generated by electric utilities and power pla nts of industrial manufacturers is disposed of in landfills; the remai nder is used primarily as admixtures in construction materials. Predic tive computer models used to assess the environmental impact of dispos al or utilization need quantitative information on ash composition and mineralogy. Typically, compositions are reported as elemental concent rations, but this data does not indicate the mineral or glass (amorpho us) phases in which the elements are contained. Such phases affect the leaching mechanisms and rates. X-ray diffraction and scanning electro n microscopy combined with energy dispersive X-ray spectrometry, commo n methods for qualitative mineral identification and particle characte rization, were used to quantify coal ash mineral compositions. Samples of industrial coal ash from a stoker boiler and a fluidized bed combu stion (FBC) system were analyzed and the results were compared to thos e for a standard sample of coal ash from the National Institute of Sta ndards and Testing (NIST 1633a). The results show that the stoker-boil er ash is similar in composition to the NIST sample. Mullite (Al6Si2O1 3) dominates the composition of the crystalline fraction and silica (S iO2) dominates the composition of the amorphous fraction. The FBC ash contains the same minerals, but with large proportions of calcium-base d phases: anhydrite (CaSO4), lime (CaO), calcite (CaCO3), and portland ite (Ca(OH)2).