Occurrences and patterns of major elements in coal fly ash under multi-acid system during microwave digestion processes

Abstract

Coal fly ash (CFA) is a typical industrial solid waste generated from coal combustion. Microwave digestion is a common measure to determine the element compositions for complex samples like CFA, of which the reagents of digestion processes largely affect the results. In this work, to explore the behaviors of major elements during the digestion process and the effects of pre-treatments, the digestion patterns of Al, Ca, Fe, K, Mg and Ti in CFA samples were studied. The results showed that hydrofluoric acid greatly enhanced the digestion when the ratio of HF went up from 0 to 0.22, of which the digestion efficiencies increased from 21.79% to 59.55% for Al, from 82.23% to 88.23% for Ca, from 50.86% to 84.93% for Fe, from 16.94% to 76.95% for K, and from 9.94% to 95.90% for Ti. Perchloric acid functioned the same as nitric acid. The most suitable schemes for major elements in CFA was (7 mL HNO3 + 2 mL HCl + 2 mL HF) for 0.25 g of samples (Al: 100.00%, Ca: 93.05%, Fe:97.58%, K: 100.00%, Mg: 81.10%, Ti: 98.30%). According to the experiment of pre-treatments and the structure analysis, it could be concluded that Fe mostly existed in the amorphous phase while K and Ti mostly existed in the crystalline phase, and Ca and Mg were mostly embedded in the Al-related crystalline phase.