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Open AccessArticle

Undifferentiated Inorganics in Coal Fly Ash and Bottom Ash: Calcispheres, Magnesiacalcispheres, and Magnesiaspheres

Instituto de Ciências da Terra (ICT), Pólo do Porto, Faculdade de Ciências da Universidade do Porto (FCUP), Rua do Campo Alegre, 4169-007 Porto, Portugal
Główny Instytut Górnictwa, Central Mining Institute (GIG), plac Gwarków 1, 40-166 Katowice, Poland
Department of Automation, Energy, Environment and Sustainable Development, Faculty of Engineering, “Constantin Brâncuşi” University of Târgu-Jiu, Eroilor 30, 210152 Târgu Jiu, Romania
University Politehnica of Bucharest, Research Center for Environmental Protection and Eco-Friendly Technologies (CPMTE), Polizu 1, Sector 1, 011061 Bucharest, Romania
Author to whom correspondence should be addressed.
Minerals 2018, 8(4), 140;
Received: 22 February 2018 / Revised: 23 March 2018 / Accepted: 26 March 2018 / Published: 2 April 2018
(This article belongs to the Special Issue Toxic Mineral Matter in Coal and Coal Combustion Products)
During a study aiming to recover strategic elements and minerals from coal fly ash and bottom ash (RAREASH and CHARPHITE projects funded, respectively, by the 2nd ERA-MIN and 3rd ERA-MIN Programs of the European Union Commission), it was found that in coal fly ash and bottom ash from Romania and Poland, several morphotypes did not fit into the general fly ash classifications, unless grouped together as “undifferentiated inorganics”. However, the combination of reflected light optical microscopy under oil immersion, scanning electron microscopy, and X-ray microanalysis (SEM/EDS) showed that many of these morphotypes not only have distinctive petrographic patterns but are also characterized by a chemical assemblage dominated by Ca, Mg, and P. In this paper, a survey of the literature is presented together with several detailed studies of samples from the RAREASH and CHARPHITE projects from which the following nomenclature are proposed: “calcispheres” for spongy Ca-rich morphotypes, “calcimagnesiaspheres” for (Ca + Mg)-rich morphotypes with visible MgO nodules and/or periclase (MgO) exsolved from Ca aluminate-silicate glass, and “magnesiaspheres” divided into “magnesiaferrospheres” for (Mg + Fe)-rich morphotypes with magnesioferrite, and “magnesiaoxyspheres” for magnesiaspheres mainly composed of (Mg + Fe)-rich amorphous material with visible MgO nodules and/or periclase. View Full-Text
Keywords: fly ash; bottom ash; petrography classification; magnesioferrite; periclase fly ash; bottom ash; petrography classification; magnesioferrite; periclase
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Valentim, B.; Białecka, B.; Gonçalves, P.A.; Guedes, A.; Guimarães, R.; Cruceru, M.; Całus-Moszko, J.; Popescu, L.G.; Predeanu, G.; Santos, A.C. Undifferentiated Inorganics in Coal Fly Ash and Bottom Ash: Calcispheres, Magnesiacalcispheres, and Magnesiaspheres. Minerals 2018, 8, 140.

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