FLY ASH FROM COAL COMBUSTION: WASTE MATERIAL OR VALUABLE SOURCE OF GERMANIUM?

Coal-fired power plants produce a major part of the energy consumed worldwide, and generate large volumes of ash and other waste products (e.g., gaseous emissions). Through the combustion process, many chemical elements become enriched in the ash relative to the original coal. For some chemical elements, the enrichments may reach levels that are close to those observed in natural ore deposits and thus, coal ash may become an economically attractive alternative raw material for mining. An example of such an element is Ge, a metalloid of strategic importance. Because of its exceptional semiconductor properties, Ge is a valuable component to electronic and optical applications. As technology advances, alternative sources of Ge will become more important. Most Ge is extracted as a by-product from ores mined for Zn, where concentrations range from 100 to 500 ppm. The element occurs also in coal, but it is not sufficiently concentrated to make extraction economical. Fly ash, on the other hand, contains much higher amounts of Ge (190-320 ppm) than coal (0.5-60 ppm). Moreover, because huge amounts of fly ash accumulate annually (57 Mt in the U.S.), 70% of which are disposed of as waste, this material is a potentially important source of Ge. We have examined fly ash produced at the Purdue University power plant, which combusts typical Illinois Basin coal (19 ppm Ge). The studied fly ash is unusually rich in Ge (up to 2400 ppm). Research has been carried out on the relationship between Ge concentration and the physical and chemical properties of the ash. Our data indicate that the Ge concentration correlates strongly with the contents of Si and Al, but not with the Fe content. This result implies that Ge is preferentially associated with the aluminosilicate glass fraction of the fly ash. Moreover, the Ge content increases with decreasing ash particle size. These observations suggest that the economic value of the studied fly ash can be further enhanced by magnetic separation and by size-separation. These two procedures should be applied before extraction of Ge, which is typically achieved through fractional distillation. Considering the very high Ge contents and the potential for further enrichment before Ge extraction, it appears that the studied fly ash could be marketed as precious commodity instead of being discarded as waste.