Study on the fate of As, Hg and Pb in Yima coal via sub-critical water extraction

Arsenic, lead and mercury are the major hazardous trace elements contained in coals. During coal combustion these trace elements release and emit to the atmosphere and cause harmful effects to the human beings and the environment. This paper is to study the fate of arsenic, lead and mercury in Chinese Yima coal by sub-critical water treatment. The experiments were performed in a semi-continuous apparatus. Demineralization, float-sink experiment and sequential chemical extraction were carried out in order to examine the transformation of occurrence mode of the three elements. The results show that with increasing temperature and residence time, the removal of As, Pb and Hg increases. At 410 °C, 15 MPa, 30 min, with water flow rate of 450 ml/h, the removal of mercury and arsenic is about 100% and 57%, respectively. It is difficult to remove lead by sub-critical water extraction. Arsenic and lead in Yima coal are associated not only with pyrite, but also with monosulfides and sulfates, other minerals including silicates, aluminum silicates etc. and organic matters. Mercury is mainly associated with pyrite (about 75%) by adsorption, and is also associated with other minerals such as clays, silicates, aluminum silicates etc. During sub-critical water treatment, arsenic associated with carbonates, sulfates and monosulfide, pyrite and other minerals all decrease, and that associated with organic matters increase. Only a little lead in Yima coal is removed by this process. Mercury associated with pyrite and other minerals decreases, and that associated with sulfates and monosulfides disappears after sub-critical water extraction.