Magnetic Properties of Low-Ca Fly Ash: A Rapid Tool for Fe-Assessment and a Survey for Potentially Hazardous Elements
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MAGNETIC PROPERTIES OF LOW-Ca FLY ASH: A RAPID TOOL FOR Fe-ASSESSMENT AND A SURVEY FOR POTENTIALLY HAZARDOUS ELEMENTS
M.J. DEKKERS* AND H.S. PIETERSEN** *Paleomagnetic Laboratory 'Fort Hoofddijk', Institute for Earth Sciences, University of Utrecht, Budapestlaan 17, 3584 CD Utrecht, The Netherlands ** Delft Technical University, Faculty of Civil Engineering, Department of Mechanics and Constructions, Section Materials Science, Stevinweg 1, P.O. Box 5048, 2628 CN Delft, The Netherlands
ABSTRACT Between 20 and 50 per cent of the total iron in low-Ca fly ashes resides in low-substitution magnetite spinel with Curie temperatures in between 565 and 5900C. A TEM study reveals that the spinel consists of aggregates of closely spaced spherules of 10 - 30 nm in diameter, which is in complete agreement with rock magnetic determinations. The low-field magnetic susceptibility correlates extremely well with total iron and fairly well with specific heavy metals. Measurement of the magnetic susceptibility may serve as a rapid tool for the evaluation of the speciation of these heavy metals in fly ash.
INTRODUCTION Magnetic characterization of Fe-bearing minerals In the present contribution so called rock magnetic methodology will be used for a magnetic characterization of low-Ca fly ash. This methodology is very sensitive and specially suited for the assessment of the iron partitioning over various mineral phases. In addition, it is rapid and usually non-destructive; no tedious sample preparation is required. Rock magnetic methods are originally designed for paleomagnetic purposes to characterize trace amounts of the magnetic iron oxides (and also sulfides) which occur in every rock. In rock samples, these iron oxides - the most important one is magnetite (Fe3O4) - are the carriers of a permanent magnetic moment, the so-called Natural Remanent Magnetization (NRM). The NRM is acquired by the rock upon the action the Earth magnetic field during the formation of the rock and, in principle, may last forever. Paleomagnetism is the Earth Science discipline which studies the behaviour of the NRM. In Earth Sciences, paleomagnetic data are used to reconstruct the movement of plate fragments through geologic time, the so called Continental Drift. Obviously, the intensity of the NRM depends on the amount of magnetite present in the rock. With the currently available sensitive magnetometers it is easy to measure the NRM of, for example, a limestone sample containing only 10 - 100 ppm magnetite. Because the properties of the NRM depend on those of the magnetite (e.g., isomorphous substitution, grain size, grain shape) a methodology is developed to characterize magnetite (and also other natural magnetic minerals) in detail to serve the Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society
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specific needs of paleomagnetism. Thermomagnetic analysis - the determination of the magnetic moment in an applied field as a function of temperature - is particularly useful to determine the amount of substitution in magnetic minerals
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