Qualitative X-Ray Phase Analysis of Lignite Gasification Ash. Illustration of a Strategy for Analysis of Very Complex Solids

Abstract

A strategy for qualitative X-ray phase analysis of complex multiphase solids is illustrated through the analysis of eight fixed-bed gasification ashes. Coal from the same lignite formation was gasified in three different reactors. Despite the differences in gasifiers and their operating conditions, all of the ash specimens had the same basic crystalline phase assemblage: silicates and aluminosilicates (merwinite, dicalcium silicates, melilite, nepheline, carnegieite and a sodalite structure phase), oxides (ferrite spinels, hematite and periclase), calcite and residual lignite minerals (quartz, plagioclase). The identification strategy relied on a bulk chemical analysis of each ash and used manual methods with the JCPDS-ICDD search manuals for the Powder Diffraction File. Grain picking, size separations, magnetic enrichment, separation by hardness, dissolution of water soluble phases, and further crystallization of the ash through heat treatment all figured in the identification strategy. Identification of more than a dozen minor and trace phases resulted from these additional methods. Several trials with an automated search/match system demonstrated the difficulties of analyses of such complex phase assemblages. The need for SEM characterization of the ashes to accompany the XRD analyses is discussed.