Migration behaviors of arsenic and sulfur from coal during washing process and their release characteristics during combustion process

Abstract

Two raw coals and their washery products were collected from two coal-washing plants of Ningwu coal field, Shanxi, China. The migration behaviors of sulfur (S) and arsenic (As) during coal washing process were investigated by the microwave digestion method together with hydride generation-atomic fluorescence spectrometry. Based on the checked mass balance of As, a sequential-chemical-extraction method was used to explore the dependence between speciation transformation and release characteristics of S and As during the combustion process of raw coal, cleaned coal, coal gangue, middling coal, or coal slime. The results show that 20%–28% of S and As in raw coal are migrated to the cleaned coal, and 46%–61% of them to gangue. Pearson correlation coefficient identifies that the inorganic minerals in samples control the migration behavior of S and As. Compared with the elements in raw coal, the proportions of organic S and As in cleaned coal increase to 30%–50%, while the inorganic S and As in coal gangue account for more than 90%, which indicates the dependence between the species of S and As in raw coal and its washery products. The relatively large amount of organic S and As in the cleaned coal obviously release together with water and volatile matters below 500 ℃. While the inorganic bonded As and S in gangue mainly release during the decomposition process of pyrite, sulfate and other inorganic minerals between 500 and 1000 ℃, which shows the consistency of S and As release characteristics during combustion of raw coal or washery product. The release rates of S and As from the cleaned coal are the fastest among all samples and the corresponding maximum release ratios are 80%–95% at 300 s and 60%–75% at 200 s, respectively, whilst their release rates from gangue are the slowest and As reaches the maximum release ratio of 40%–45% at 300 s but S doesn’t get to the maximum release ratio even at 600 s. The release rates of S and As from the middling coal or raw coal are between the cleaned coal and gangue. The different release rates of S and As during the sample combustion are mainly depended on their speciation distributions in nature.