Release characteristics of arsenic, selenium, lead and transformation of minerals during ashing process of coal

In this paper, two kinds of coal (Baiyinhua lignite and coal using in Ezhou Power Plant) were selected to study the occurrence of heavy metals including arsenic (As), selenium (Se) and lead (Pb) in coal and their release behaviors during the ashing process. The influence of ashing conditions on the mineral conversion was also examined by the combination with the changes of minerals in coal. The burn-out temperature of the coal was determined from the thermal weight loss curve by extrapolation method, and then the coal sample was ashed according to the National Standard Method of China. The obtained ash samples were characterized by XRD, XRF, and TG-DTG to analyze the change characteristics of coal minerals at different ashing temperatures. The content and occurrence of As, Se and Pb in coal samples were extracted by sequential chemical extraction method. The heavy metals in the ash sample were extracted by HNO₃ + HF, and the heavy metal content in the extract was detected by Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The results show that the heavy metal in BYH coal is mainly As in the form of sulfide bound state, which leads to the increasing volatility of As with temperature easily. However, the heavy metal in EZ is mainly sulfide-bound Pb, which makes it easy to release with temperature. Se in coal mainly exists in the form of organic bound state and sulfide bound state. During coal ashing, kaolin is gradually dehydroxylated into metakaolin and finally converted into mullite; pyrite is oxidized to form hematite; gypsum is dehydrated to form anhydrite. The release rate of heavy metals is greatly affected by the combustion temperature during the ashing process. High content of heavy metals in the sulfide bound state results in high release rate with increasing the temperature.