Application of advanced analytical techniques to elucidate structural and chemical evolution of combustion-generated ash

This work reveals the characteristics of the ash obtained from the combustion of paper mill sludge. A model of the ash pellet was synthesized from a mixture of oxides of Al, Si, Na, K, Ca, Mg, and Ti using TG furnace at 1500 degreesC. The ash pellet was doped with heavy metals such as Cr, Cd and Pb prior to the combustion in order to establish their behavior during ash formation. The atomic force microanalysis (AFM) of the ash (10 x 10 mum size) revealed a smooth and continuous outer surface topography. The maximum vertical height variation of the surface was 3.8 nm. The ash material contained a compact, dense rigid internal morphology as evidenced from the scanning electron microscopy analysis. The ash material contained crystalline mullite, corundum and alumino silicate glass phases. The Al-27 and Si-29 solid-state nuclear magnetic resonance spectra with magic angle spinning of the ash also supported the formation of the above three phases. It was shown by the presence of both AlO6 octahedral peak (10.0 ppm) and AlO4 tetrahedral peak (50.0 ppm) for aluminum present in these compounds. Silica showed a characteristic SiO4 tetrahedral peak at - 99 ppm, which is due to the presence of SiO4 group in alumino silicate matrix. The final ash material was composed of alumino silicate glass with embedded corundum and mullite crystals. The electron probe microanalysis showed the inclusion of Cr into the corundum phase, which lead to the Cr immobilization against leaching. The outer surface of the ash was found to be rich in alkali oxides compared to the inner core in which the other two heavy metals Pb and Cd are enriched. The final pellet was considered non-hazardous. (C) 2001 Elsevier Science Ltd. All rights reserved.