Effect of washing process on mercury migration and emission of Ningwu coal

Based on the mercury (Hg) mass balance, a sequential-chemical-extraction method and the thermal analysis technique were used to investigate the occurrence forms of mercury in coal and the effect of washing process on the Hg migration and emission characteristics using Ningwu coal and its washery products. The experimental results show that the ash content and sulfur content reduce in the cleaned coal and Hg is enriched in the gangue, causing an Hg redistribution in the washery products. The linear correlation coefficients of Hg content with ash and sulfur are 0.89 and 0.99, respectively. Hg mainly exists as an inorganic bound state in the raw coal, so its migration behavior is determined by the transfer of inorganic minerals during the washing process. Gangue shows the highest mass increase in the Hg content by 322.8% compared to the raw coal, while the content of Hg in cleaned coal decreases to 40% of that in raw coal with the removal efficiency of 56.4%. The difference of mercury enrichment degree in coal washery products is closely related to its occurrence form in raw coal, in which the proportion of sulfide-bound mercury in gangue is as high as 56.6%, while the organic bound mercury in cleaned coal and slime is higher than that in raw coal. Furthermore, Hg in all samples has been completely released at 650 ℃ during pyrolysis process. Hg has the highest release ratio in slime and the lowest one in gangue at the same pyrolysis temperature. The release characteristics of mercury in coal is also closely related to its occurrence forms, which determines the Hg release behavior at different temperatures. The organic bound mercury escapes in large quantities at 300 ℃ with the decomposition of organic matter, while the decomposition temperature of sulfide bound mercury mainly concentrates at 400−600 ℃.