| Tannery sludge is a complex and toxic industrial waste that has been included in the "National List of Hazardous Wastes".China is one of the world’s major leather production centers,producing nearly 1 million tons of dry tanning sludge each year.Sludge usually contains a large amount of heavy metal elements.Effectively fixing heavy metals in sludge is a necessary prerequisite for the safe disposal and resource utilization of sludge.Rapid pyrolysis can effectively treat a large amount of sludge and recover resources such as bio-oil and bio-char,which is an ideal means for the resource disposal and utilization of sludge.On the one hand,the enrichment characteristics,chemical form distribution and bioavailability of heavy metals in biochar are different under different pyrolysis conditions.In order to avoid the release of heavy metals into the environment and cause secondary pollution,the distribution of heavy metals during sludge pyrolysis was studied.Regularity and morphological transformation are crucial.On the other hand,inherent metal elements(such as Cu,Zn,Cr,etc.)in sludge will affect the yield and composition of pyrolysis products.Taking heavy metal-removed sludge and original sludge as the research objects,the volatile matter content and component distribution under the same pyrolysis conditions were detected by GC-MS.The main findings of this paper are as follows:(1)During the pyrolysis process,heavy metals will volatilize to different degrees,and the volatilization rates of heavy metals in the two atmospheres have different trends.Compared with oxidizing atmosphere,heavy metals are generally more easily volatilized in reducing atmosphere.The proportion of heavy metals Cu,Ni,Cr and Mn in bio-oil is less than 1.5%,and the proportion in biochar is all over 80%,indicating that they are not easy to migrate during the pyrolysis process and are mainly distributed in biochar.At 800℃,the volatilization rate of Zn and Pb is about 40%,showing significant volatility.With the increase of temperature,the proportion of Pb in the biooil remained stable below 1.5%,but the proportion of Pb in the gas increased significantly,and Pb migrated from the biochar to the gas during the pyrolysis process.Zn is distributed in both bio-oil and gas.(2)The results of morphological analysis showed that Cu,Zn,Ni and Mn in the tannery sludge were stable,and the percentage of F5 in the residue state was about 84%,97%,95%and 93%,respectively.With the increase of the pyrolysis temperature,the relatively stable components F4 and F5 increased.The pyrolysis had a positive effect on the immobilization of heavy metals in the sludge,and the pyrolysis temperature played an important role in the immobilization process.The redox atmosphere also has a certain influence on the form transformation of heavy metals,among which heavy metals Pb,Cu and Ni are more stable under nitrogen reducing atmosphere.Infrared spectroscopy and XRD patterns showed that heavy metals would react with the components in the sludge during the pyrolysis process to generate stable heavy metal compounds,which were mainly immobilized in biochar in the form of CuMn2O4,CuFeS2,ZnCr2O4,ZnFe2O4,Cr2O3,MgCr2O4,etc.The XPS analysis results were consistent with the change of chromium crystal phase in XRD,and it was also found that the oxidation of trivalent chromium(800℃)could be effectively avoided by treating the tannery sludge under nitrogen atmosphere.(3)Taking the original tannery sludge as the research object and removing the heavy metal sludge(H3PO4 pickling)as the control,the influence of the inherent metal elements in the sludge on the pyrolysis products and volatile matter of the sludge was investigated.Removing inherent metal elements can improve the tar yield and inhibit the biochar yield in the sludge pyrolysis process.At all temperatures,the oxygenates of the acid-washed samples were lower than those of the original samples,and the concentration of aromatics increased with the removal of metal elements. |
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