The samples are ashes collected from electrostatic precipitators (filters) of a Bavarian power plant. Ashes from two different fuels were investigated:

	ASH1 from coal
	ASH2 from a mixture of coal plus sewage sludge

Techniques

Magnetic extracts of the ashes were embedded in resin and cross sections were polished using  diamond pastes with the maximum polish obtained with 0.25 µm paste.

Polished cross sections allowed the internal structure of the spherules to be examined. The cross sections were investigated using a reflected light microscope (Leitz Ortholux Pol), where oxides magnetite and haematite can be distinguished using the usual optical property data (e.g. Craig and Vaughan, 1994). For some of the images, a ferrofluid was placed on the sections, which also allows the distinction of highly magnetic magnetite (which is covered by the ferrofluid) from weakly magnetic hematite and non-magnetic phases.

Scanning electron microscopy (SEM, with a Zeiss DSM 960, Institute fur Geology, University Munich) was conducted both on the cross sections as well as directly on the ashes, which were stuck on carbon pad sample holders. For most of the images, the electron microscope was used in back-scattered electron mode (BSE). These rough surface BSE pictures show the grain morphology and also differences in element composition. Relatively bright areas on grain surfaces are those with higher average element number than areas with lesser brightness (essentially differences in Fe-concentration in the grains). Qualitative analysis of the composition was done by energy dispersive X-ray analysis (EDX). This method has a spatial resolution of some 3-5µm. With the EDX-system used,  it was not possible to determine oxygen content.

Results

Both ashes contain mostly spherules and can not be clearly distinguished from each other by microscopy. Many examples of these spherules are shown. However, the magnetic parameters of the ashes showed differences, most notable in saturation magnetization which is approximately 3 times higher in ASH1 than ASH2 (ASH1 has a magnetite content of ~ 1.5 % by weight and ASH2 ~0.5 %).

The magnetic minerals were magnetite and hematite, very often in the form of an intergrowth like that observed from "martite" lithogenic grains. Titanium concentration was very low.

The spherules have either a smooth or a structured surface. Their size is from less than 1 µm to about 120 µm, with most of the grains smaller than 50 µm.

Spherules consisting of Fe-oxides have a structured surface, often resembling "brain convolutions" or consisting of small crystals.

The spherules with smooth surfaces were of mostly Fe-poor and dominated by Al, Si (oxide) composition, indicating they are mostly silicate glass.

Al & Si-rich spheres sometimes contained highly symmetrical skeletal iron oxides in their interior, which sometimes touched the surface and consequently formed surface structural features which could be observed under backscatter mode in the SEM. Al, Si-rich spheres may also contain dendritic iron oxides with 90° angles between primary and secondary branches. The secondary braches have a typical spacing of 0.5 µm.

A few spherules were hollow and contained "baby" spherules themselves. These are called plerospheres after Fisher et al. (1976).

This study was a part of my diploma thesis (Matzka, 1997) supervised by N. Petersen.

References

Craig, J.R & Vaughan, D.J. (1994). Ore microscopy and ore petrography. John Wiley, New York.

Matzka, J. (1997). Magnetische, elektronenmikroskopische und lichtmikroskopische Untersuchungen an Stäuben und Aschen sowie an einzelnen Aschepartikeln. Diploma Thesis, Institute for Geophysics, University Munich