Transformation of Heavy Metals during Pyrolysis of Predried Dyeing Sludge

Heavy metals in sludge may cause serious pollution and restrict the industrial application of sludge heat treatment. In this paper, predried dyeing sludge (PDS) char samples were prepared on a fixed bed in a pure nitrogen atmosphere. X-ray diffractometry, microwave digestion, and inductively coupled plasma-atomic emission spectrometry were used to measure the content, speciation occurrence, and the transformation of heavy metals during heat treatment of PDS samples. The results show that heavy metal elements such as Cr, Cu, Hg, Ni, and Pb in PDS both in raw PDS and derived char samples are mainly dominated by hydrochloric acid soluble speciation and insoluble speciation, while the ammonium acetate soluble speciation is very little and the water soluble speciation is negligible. Part of the hydrochloric acid soluble Cr is volatilized when pyrolyzed at 400-600 degrees C. Cr is mainly fixed in the PDS char as the insoluble speciation and hydrochloric acid soluble speciation above 600 degrees C. One part of the hydrochloric acid soluble Cu is volatilized and the other part transforms to the insoluble speciation at 400-500 degrees C. Cu is mainly fixed in the PDS char in the insoluble state above 600 degrees C. A small amount of Hg transforms from the hydrochloric acid soluble speciation to the gaseous speciation. Ni is observed to transform from the hydrochloric acid soluble speciation to the insoluble speciation below 700 degrees C, while it transforms from the insoluble speciation to the hydrochloric acid soluble speciation above 700 degrees C. The higher the temperature is, the more the ammonium acetate soluble Pb and the hydrochloric acid soluble Pb released to by volatilization as gaseous Pb. The fixing ratio of Cr, Cu, and Ni is proportional to the pyrolysis temperature, and the fixing ratio of Hg and Pb is inversely proportional to the pyrolysis temperature. PDS has a high fixing ratio of heavy metals at 700-800 degrees C, which indicates the best temperature window for heavy metal fixing. The present results may be helpful for the prevention and control of heavy metals in PDS.