Characteristics of ferrospheres in fly ashes derived from Bokaro and Jharia (Jharkand, India) coals

•Fly ash from Bokaro and Jharia (Jharkand, India) coals•Ferrospheres were studied using a combination of analytical techniques.•Fe contents are normal, ferrospheres are low, and iron is not a potential contaminant of groundwater.•Fe-crystallites result from iron micro-plumes in the aluminosilicate melt.

Coal burning power plants in the state of Jharkand, India, are facing the problem of fly ash landfilling and their economic and environmental impact. However, fly ash may be used in civil engineering constructions, including as geoliners for municipal wastes landfilling, however in this case the groundwater contamination should be taken in consideration.In this work a combination of analytical techniques is used to study the nature, composition and potential environmental impact of Fe-bearing morphotypes (ferrospheres) in fly ash from thermal power plants fed with coals from Bokaro and Jharia coalfields (Jharkand, India).The results show that the feed coals are sulfur-poor and ash-rich, dominated by quartz, clays and minor portions of Fe-bearing carbonates, such as siderite. Pyrite was not identified. Although iron is present in the fly ashes in significant proportions (from 2.7 wt.% to 4.5 wt.%), equivalent to an Fe2O3 content ranging from 3.5 wt.% to 5.8 wt.%, mineral phases such as magnetite and hematite are only present in minor proportions, or below detection limits of the XRD analyses. Iron in the ferrospheres occurs as massive or dendritic crystals, or as finely dispersed crystals trapped inside a glassy aluminosilicate matrix resulting from the release of iron oxide plumes into the aluminosilicate melt. In addition to these phases, iron also occurs as a component of the glass that makes up most of the fly ash materials. Finally, the contaminant potential of groundwater by the fly ash iron is negligible.