Migration and transformation of heavy metals during oily sludge pyrolysis

During oily sludge pyrolysis, the heavy metals contained in oily sludge may migrate to pyrolysis gas, pyrolysis liquid, or remain in the pyrolysis residue. In order to explore the migration and transformation behavior of heavy metals during oily sludge pyrolysis, the effects of pyrolysis temperature and residence time on the distribution of heavy metals in the solid-gas-liquid phase products and their morphological distribution in the pyrolysis residue were mainly investigated. In addition, the leaching characteristics of heavy metals in the pyrolysis residue were analyzed, and their ecological risks were assessed. The modified BCR three-stage sequential extraction procedure and risk assessment index method were adopted to conduct morphological analysis and risk assessment on heavy metals in the pyrolysis residue. Later, the pyrolysis residue was analyzed by means of elemental analysis, X-ray fluorescence spectroscopy, and Fourier infrared spectroscopy, and the influence mechanism of various factors on the migration and transformation of heavy metals was discussed. The results indicate that:(1)During oily sludge pyrolysis, heavy metals are mainly distributed in the pyrolysis residue, and less than 3 % of heavy metals are distributed in the pyrolysis liquid and pyrolysis gas. (2)Low temperature has little effect on the migration of heavy metals, and the migration rates of Cr, Cu and Pb rise sharply at 700 ℃. (3)Pyrolysis enhances the stability of Cr, Cu and Pb, but is not conducive to the immobilization of Ni and Zn. (4)Pyrolysis reduces the leaching concentrations of Cr, Ni and Pb, but activates the leaching activity of Zn, which results in an increased risk for Zn from low to medium level and the immobilization effect of heavy metals is closely related to the content of inorganic minerals.