Leaching behaviour of metals from post-underground coal gasification cavity residues in water differing in mineralization

•Ashes and chars from quenched reactors after three UCG experiments were tested.•Waters with diverse mineralization may have different leaching abilities.•Eluted metals were divided into groups based on their mobility to water phase.•Some toxic elements (Cr, Pb, Cd) may migrate to water from UCG hard coal residues.

The aim of this study was to examine whether water entering the post-reaction area after underground coal gasification (UCG) leaches metals from the post-processing waste in the cavity to such a degree that it can pose a threat to the environment.In the studies, ashes and chars arising from hard coal and lignite gasification have been used. Leaching tests have been performed with the usage of deionized water and two types of mine water that greatly differ in mineralization. The physicochemical composition of the obtained eluates has been analysed. The coefficients of dispersion (distribution), kd, have been estimated, which allowed determination of the mobility of metals under test and thus their ability to leach to the water phase. Metals present in the process of UCG have been described in terms of volatility and divided into three groups. In addition, the concentrations of metals in the eluates, depending on the leaching medium used, have been investigated.It has been found that the majority of tested elements tend to be leached in a stronger degree from ash residues than from chars. In comparison with the residues from hard coal, the residues of gasified lignite show the higher relation to the water phase. The presence of such elements as Cr, Pb or Cd in eluates may suggest that toxic metals are able to leach to water from the residues of the gasification process and a non-negligible risk of environment contamination might occur.

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