Date of Award
Master of Science
Since a substantial amount of coal combustion byproducts (CCB) are produced each year, generating value-added product from fly ash, which is a major constituent of these CCBs, has been an important area of research for several decades. Natural magnetite (NM), which is used to maintain dense medium slurry pulp density in coal preparation plants, has a current market value of more than $200 per ton. The use of fly ash derived magnetite (FAM) as an alternative to natural magnetite has potential benefits for dense medium processes, such as lower cost, greater stability at low medium density, more efficient delivery systems. This study developed a suitable processing scheme to extract high-grade (> 96%) magnetite from fly ash generated from burning high sulfur coal, and investigated the suitability of the FAM product for dense medium application in coal preparation plants. A classifying cyclone was utilized in the process flow sheet for the pre-concentration of FAM in its underflow stream, which was enriched to high grade FAM by a single stage wet magnetic separator of low intensity (~1000 gauss). A statistically designed experimental program was utilized to maximize the magnetite grade and recovery achieved from the above mentioned flow sheet. The FAM product particles had a slightly coarser particle size distribution than the NM particles. In addition, the FAM particles were found to have a spherical shape; but about one unit lower specific gravity in comparison to the NM particles. However, the F5 Stability Index of the resulting FAM product was found to be in the desired range of 30 to 40 for its suitable application as a dense medium. The coal cleaning performance obtained from a 0.15 m diameter dense medium cyclone using dense medium prepared from both of FAM and NM, were quite similar. However, the effective separation density (SG50) obtained from the FAM-based dense medium was significantly different from the medium density; this may need further investigation in future. A preliminary economic analysis, conducted for a hypothetical mini-plant having a fly ash handling capacity of 100 ton/hour, indicated the cost of FAM extraction to be nearly $5/ton. The cost assumes that the FAM extraction plant is located at the fly ash producing utility site and does not include the cost of thermal drying that may be required to reduce the moisture content of the FAM filter cake produced at the FAM plant. A preliminary civil engineering study conducted to investigate the effect of FAM extraction on the compressive strength property of the non-magnetic flyash (left behind after FAM extraction) failed to produce a conclusive finding. The specimens prepared using 10% and 30% fly ash replacements indicated that the compressive strength does not change due to FAM extraction. However, the specimens using 20% fly ash replacement indicated that compressive strength does change due to FAM extraction. Hence, a more detailed study is recommended to investigate this discrepancy.
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