Leaching of various metals from coal into aqueous solutions containing an acid or a chelating agent

Twelve coals were subjected to leaching procedure with an aqueous solution containing an acid or a chelating agent, such as EDTA. The susceptibility of each metal in coal for the leaching was roughly divided into three classes: largely leached (Ca, Mg, and Mn), moderately leached (Cu, Fe, Pb, and Zn), and a little leached (Al, Co, Cr, and Ni). This tendency was common in the leaching by HNO3 as well as in the leaching by EDTA. The leaching of each metal reached a pseudo-equilibrium value within 10–30 min. The metal leaching was decreased when the concentration of HNO3 or EDTA was lowered. However, even a low concentration of EDTA (0.1 mM) had an appreciable ability of metal leaching. For several metals (Ca, Cu, Fe, Mn, and Mg), there were fairly good correlations between the leaching amount and the content in coal.