Investigation of Metal and Trace Elements of Cenospheres from Lignite High-Calcium Fly Ash (Thailand)

Sorachon Yoriya,Phattarathicha Tepsri

doi:10.3390/w13202935

Abstract

High-calcium fly ashes contain a large content of small particles including cenospheres of chemical constituents known to be similar to fly ash and the parent coal. Coal fly ash contains metal and trace elements that may leach out during disposal or utilization. This work aimed to understand an overview of cenosphere characteristics relating to fly ash and leaching study. To our knowledge, this is the first report on metal and trace element leaching of cenospheres separated from high-calcium (28.9 wt.%) class C fly ash produced from the Mae Moh coal-fired thermal power plant in Thailand. In this study, the cenospheres were separated from fly ash by a wet separation process (sink-float method) using water as medium. Physical properties, morphology, chemical composition, the mineral phases of cenospheres and fly ash have been characterized. Extraction was carried out by acid digestion; the selected metal and trace elements in this study are Mg, Al, Zn, Pb, Cd, Cr, and Cu; the obtained environmentally available concentrations of cenospheres were analyzed in comparison to those of fly ash. The concentrations of Cu, Cr, Pb, and Cd elements of interest in the leachates obtained from the toxicity characteristic leaching procedure (TCLP) showed the tendency to decrease in that order. All the elements were found below the permissible limit values regarding Thailand soil quality standards. Association of the heavy metal trace elements in cenospheres and fly ash was discussed in in terms of physico-chemical-geochemistry correlating with the leaching concentrations.

Highlights

Coal contains significant quantities of minerals and a high concentration of trace elements
The toxicity characteristic leaching procedure (TCLP) method employed in this study has shown a significant role in determining the leachability of the heavy metal elements from cenospheres
This work presented the separation of cenospheres from lignite high-calcium fly ash via the sink-float method using water as the medium

Summary

Introduction

Coal contains significant quantities of minerals and a high concentration of trace elements. Bouska and Pesak determined statistical characteristics of all the elements in thousands of coal samples and found that arithmetic mean values for Hg, Cd, Pb, As, Cu, and Cr are 0.13, 5.6, 11.1, 33.4, 35.3, and 54.5 mg/kg, respectively [2]. Lignite coal normally showed the trace elements in higher concentrations, e.g., 72 mg/kg of As, 95 mg/kg of Cr, and 254 mg/kg of Cr [3,4]. Various trace elements are emitted associated with surface formation of fly ash particles during coal combustion [5,6,7]. Concentrations of the trace elements were found in fly ash approximately 4–10 times higher than those in the parent coal [5]

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